How do you have forum indicate how many days ago thread was started?
 

On some forums like forums.digitalpoint.com, it is indicated beside the thread name how many days old the thread is. How do I get that feature?

That's the creation date or first post date. There are a couple of mods here to do that (in fact this question was just asked a couple of days ago). Try a search on "creation" or "post*" in "all mods" "titles only"

when I try to install it, it says the plugin is not compatible. also it shows the date, any chance of finding a plugin that shows how many days ago it was created

Showing how many days ago versus the actual date is something you will need to change in the vbdate function in the plugin (look it up in the API). Any problems you have with installation need to be addressed in the modification thread.