I have already changed the default text in 'description' box in the enter_bug.cgi page by editing the create.html.tmpl template but I am unable to find the template where I can edit the post_bug.cgi page. Does anyone know where I can find this?
I am using Bugzilla version 3.4.2
Look for the INCLUDE of global/textarea.html.tmpl at line 250 of template/en/default/bug/edit.html.tmpl. You should be able to add a defaultcontent parameter to get the results you want.
You might also want to add some content in template/en/default/list/edit-multiple.html.tmpl for when someone is changing multiple bugs.
To find stuff in the Mozilla code, it's usually best to examine the HTML for the page you're looking at to find some text that is unique to the context, then go look for that. In this case, searching for label for="comment" would have led you to the correct places.