its on a stalk on the left side of the steering wheel. you slide the front of it to the on position (left) and then just push the button on the tip of the stalk to activate the cruise control

 

Things to look for on the service history and during visual inspection...
1) timing belt- if its not beendone yet, get it done @90k est. $650 w/parts
2) cam adjuster gaskets/valve cover gaskets- if they haven't been replaced, look for signs of seepage aroung the cam adjusters. These will eventually need replacing as well.
3)visually inspect the front outer CV boots for cracking. This job is a bitch, but it'll need to be done soon if not already.

Just my 2 cents from my 1st year and a half in a 01 A4 2.8Q... overall, awesome car though.

 

Weak, luckily I don't ever need to use cruise..