The card should have included some type of adapter for that... Do you know what kind of power supply? You need to make sure it has sufficient wattage, but more importantly, enough +12v amperage. Minimum recommended for the 550 Ti is 400w. But like I said, amperage on the +12v is very important too.

Is this your HP Pavilion p6-2114 model?
http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c03288944&lang=en&cc=us&taskId=135&contentType=SupportFAQ&prodSeriesId=5187028&prodTypeId=12454

That is not the exact one but yes same power specs, the guy at Best Buy told us what to buy and even showed my son a image of where and how to put it in, just not how to plug the power supply cord in.  Did he not give us the right stuff??

ok, this should be the link to our computer, a p6-2114 not the p6-2114d
 

 
hope I did this right :>)

That still only has a 300w powersupply.... 
A dual core Intel I3 2110
Since that CPU is only a 65w draw, a 300w PSU "might" work for a 550Ti... but thats a big "might".
It could just as likely have problems or cause the PSU to fail, in either case requiring a new powersupply.
  
Since you are the "parent" buying this stuff, may I ask what is this computer to be used for? 
 

Well....
 
To hook up the videocard, use the power Adapter that came in the box with the card
Each end (2) needs to be connected to a unused 4pin molex connector off the power supply. If you do not have 2 of these that are available, you will need to go to Radio Shack and get a Molex splitter.

 
Be aware that if your son has videocard problems and/or the computer wont start, you will need to replace the powersupply with something like this http://www.newegg.com/Product/Product.aspx?Item=N82E16817371045 at a cost of between 30-$50 
 
I cannot say that the videocard is going to work, using only a 300w PSU. So just keep that in mind 
 

As stated above, the 550 Ti needs a 6-pin PCIe power connector plugged into it..There may/may not be one coming  from the power supply..If there is, just plug it into the 550 Ti..If there is not, you need to use the dual 4-pin molex to single 6-pin PCIe adapter that should have came with the 550 Ti (providing the power supply has 2 available 4-pin molex connectors that you can plug them into) or go buy one..If your power supply does not have the 6-pin PCIe connector or the 2 available 4-pin molex connectors, then you need to buy a power supply that does..A 500 watt should work fine and they're not that expensive.

For a start. Bad move going to Best Buy. They usually know nothing and prey on the non technical folks just to get that sale. I work and have owned many HP's systems. The current systems power supply wise do not offer molex connectors for power. They also stick in crappy ones unless you get the 460w or 600w versions. Those are on higher end models and good quality. You will need to replace the power supply. I suggest going with this one. http://www.newegg.com/Pro...x?Item=N82E16817139026 It is a good quality unit and perfect for that system.
 
P.S.
 
I have a brand new 600w power supply that HP sent me under warranty. I do not need it. Something else was wrong with my system at the time. I am sure we could work a deal out. I also have pics of it. PM if interested.

That might be best to take it back. Just a warning. Their prices are much higher and they are going to hit you with a install fee. That is not cheap either.

Glad you figured it out this far.

In my experience, the sales people at Best Buy tend to not know much about PC stuff. I remember a long time ago, I was at Best Buy, and I was overhearing one of the sales people talking to this man looking for a TV Tuner card, and a graphics card. Back then, they had ATI and 3Dfx Voodoo cards that had TV tuners built in... Instead of showing the man those (on the rack right behind her), the sales girl sold him separately a TV Tuner card and a graphics card. The man could've saved a LOT of money just going with one of the 3dfx or ATI cards. I don't know if she didn't know about the 3dfx and ATI cards, or if she was just maximizing her commission. Either way, it reflected poorly on Best Buy IMO.
 
If you've got a Fry's or Microcenter around where you live, I highly recommend shopping there instead. Not only are they more knowledged in PC hardware, but their prices are MUCH better than Best Buy, and they won't screw you over like used car salesmen.