Using joomla 1.5.23 that module does exist (modules/mod_weblinks) so I put it(jimport('joomla.application.component.model') in modules/mod_mainmenu/helper.php and it worked
I am at a loss still after reading your advice and the follow up posts. Where exactly in line 14 of the helper.php file do I insert the code "jimport('joomla.application.component.model');" or where exactly is "at the top of the code" as you indicate in your post?
I had an error on page modules \ mod_lofarticlescroller \ libs \ source \ content \ source.php, and entering: jimport ('joomla.application.component.model') is solved for that page, thanks