I have found the answer.
This is what to do:

Q: When I select a search result and click the Download button, nothing happens! What's up with that?
A: You may have selected "Always use this answer" on a dialogue box. In order to correct it, go to Tools > Options > View > Popups > Revert to Default. On a mac, go to LimeWire > Preferences > View > Popups > Revert to Default.

It works i tried it