Use your quick menu buttons. Click a quick menu button. Then go to tires. Then select Inters or Wets. Next pit stop nerdy the engineer will have spanky, curly, larry, moe, and the other guy, put them on your car.
You can set quick menu buttons in controls, I have mine set to the D-Pad on the PS2-USB controller and also on my DFGT the rare time I can find room to play with it.