I removed Rhythmbox and installed Clementine. Now, the Music lens is empty. How can I search for music without Rhythmbox?
Out of the box - the only current media-player scopes available are banshee & rhythmbox.
Basically you cannot use clementine together with the music lens to search for music in Ubuntu 12.04 - however if you are an 12.10/13.04 or 13.10 user you are in luck.
For 12.10 & 13.04 users, the scope-packagers maintain a PPA which have the following media-player scopes in their package
unity-scope-audacious Audacious Scope for Music Lens unity-scope-clementine Clementine Scope for Music Lens unity-scope-gmusicbrowser GMusicBrowser Scope for Music Lens unity-scope-guayadeque Guayadeque Scope for Music Lens unity-scope-musique Musique Scope for Music Lens
For 13.10 users, this has been split into individual packages and are available in the repository: