There should not be the need for the addons folder - instead you can add the source projects to your workspace.
If you want to add binary jars nonetheless, you best add them to your target platform - see here for some documentation on this.
I want to add the prexisting addon jar file org.openhab.ui.habmin_0.1.6.jar (that I downloaded from here) into a working openhab2 in the IDE. I followed these instructions
Window -> Prefernces -> Plug-in Development -> Target Platform shows a Target Platform dialog box with an item selected in bold: openHAB Target Platform (Active) - /launch/openhab.target.
Selected Edit -> (Locations tab) -> Add -> Directory, and added /home/avner/openhab2-master2/ws/tmp1 where I placed the jar file.
After that I’m seeing the habmin jar file (see snapshot)
Following the instructions to execute openhab I see messages in the Console pane, and also http://localhost:8080 responds as expected, but http://localhost:8080/habmin doesn’t come up.
I’m probably importing the habmin jar addon improperly. What should I do to import this jar file into the eclipse, so that it’s taking effect in the project?