Launch Hammer++. Go to Tools then Options on the menu bar at the top of the window.
Under the Game Configurations tab, look to where it says Game Data Files. There is probably only one entry there for you.
Click the Add button to the right, then go to your TF2 install directory then under Team Fortress 2/bin, look for tf2-abs.fgd. Add that.
Then restart Hammer++ and see if that works.