hi, iam using ver:3.9.0 on windows server. i want to add addsense ads to my user signature. can anyone help me about it.

In this area you can only use bbcode. No html and no javascript

then is there an easy way to add ads to forum?

Sure, first ad a new Bannerposition - admin > banners(look at the number that you for this position), make a new bannercustomer > select your new position > choose script and insert your adsense script.
Ther are more possibilities to insert it in the forum. One simple solution for showing on top and bottom:
Open your templates/XXXXXX/home.html and insert the litte code in the Page section <!-- PAGE i -->

Otherwise you can put in the pages/forum/index.php.
Try to find the right position for you.

thank you