2
@ robertelo
Your question is about basics in XOOPS customization, so I'll answere a general answere ...
(almost) all the module's apearance acheived via it's _modules_/_your-module_/templates/whatever.php files.
Block apearance via it's _modules_/_your-module_/templates/blocks/whatever.php ....
So go there and edit the file you need to (you'll probably understand from the /templates/blocks/ file names which block file should be edited ...
Since Classifieds module need the user to submit the ADD from within a category, adding it to the block may be a bit useless
Don't forget to update the module afterwards (via Site Admin ==> modules admin)
Advice: read a beat @
Xoops FAQ