Where is this wireless on/off button of my compaq presario CQ42. I cannot connect to wifi....

Maybe you don't have your wireless turned on. Look onboth sides and in front for the Wireless Switch and turn it on.

Also once the wireless switch is turned on there are keystrokes that can togglethe wireless on or off. Normally you would Press and Hold the FN functionkey located at the lower left of the keyboard, then hit the correct F1 thru F12key that toggles the wireless. The correct F1 thru F12 key to hit willhave the wireless signal icon )))) on it. Or it might have the icon of a towerwith the wireless signal icon )))). Normally the screen will displayWireless On or Wireless Off when you toggle the wireless.
Also check your software that you are using to control the wireless. There are two ways to control the wireless. 1) Use the software that came with the adapter or 2) Let Windows (Windows Zero Config) control the wireless. Check them to make sure you don't have the Radio OFF.
OK I just checked your manual and it says to Hit the F12 Key to turn the wireless on and off.A link to your manual is below.
http://h10032.www1.hp.com/ctg/Manual/c02100456.pdf