Hope someone here can help ...
 
I got the arc tool and returned to the base. When the start point loaded to go to the power unit room to use the arc tool I was shown holding a regular weapon and the arc tool listed in the HUD display as "unbound". I can't find any reference in the controls keyboard set up for non-titan mech weapons or tools. I did a verify files check, restarted from the last save point .... no joy. So, I'm dead in the water. This is my 2nd playthrough and didn't run into this problem with the first one. 

sinned.
Should be C if I remember correctly.

 
Problem was self inflicted. I had reassigned the "C" key from the default left CTRL key to toggle crouch on the 2nd campaign play through not realizing the C key is used in MP. For whatever reason reassigning the MP "C" key for the campaign created a conflict resulting in the arc tool key shown as "unassigned". A guy in the TF forum set my straight on this and resetting the campaign control key assignments to default corrected the conflict, with the C key now shown as the arc tool access key. Bottom line - you can't reassign the C key in campaign mode in this game as I have done in other games.