Lipra,
just to make sure I'm up to date I understand that the RTMM sceneries showed in your Scenery Library BEFORE you installed the Ibiza scenery. After you installed that scenery then nothing else is visible in your Scenery Library except the Ibiza entry? Nothing else shows including all the default scenery folders that were there before you installed Ibiza?
If this is the case you can try this:
Navigate to your Scenery.cfg file. For XP this is C:\Documents and Settings\(USER NAME)\Application Data\Microsoft\FSX. Back up the Scenery.cfg by making a copy and rename the copy to Scenery.cfg.ORG, then delete the scenery.cfg file
Start FSX and it will build a new Scenery.cfg file that should put the scenery back into your Library. If it all goes south you can navigate back to the Scenery.cfg and delete it and rename the copy Scenery.cfg and your back to square one.