Are you tryin to open earlier verion of catalog?
If this is the case then locating the Lightroom 3 catalog file and opening it in Lightroom 4 ( right click and choose program) will prompt you to upgrade the catalog. This will fix the issue.
If let me know incase it doesn't worked.
I had lightroom 3, But uninstalled it and installed lightroom 4 thinking that would fix it. I tried doing what you said and I can't find the program option you are talking about. I'm on windows if that helps
Hi Please perform the following steps:
1. Go to your catalog file (.lrcat) and make a backup (just in case)
2. Open Lightroom – “OK” the “assertion failed” messages
3. Edit > Preferences > General > Default catalogue > choose “prompt me when starting Lightroom”
4. Close Lightroom, then re-open it
5. You should get dialogue box “Select catalogue” – choose “Create new catalogue”
6. This brings up dialogue box “create folder with new catalog” – choose a name for your new catalog – “save”
7. Lightroom “reopens” – File > Import from catalogue > choose the last catalog you were using before the problem (file type = “LRCAT”) – watch while it imports all your image data – takes a few minutes (for 4''' images only) – its worked for me!
8. Edit/Catalog settings - set Lightroom to backup every time you start (as a fail safe should you reopen LIghtroom – so it backs up and then close it again before going?).#