To split images into the two different events, you would not use catalogs. You can use either keywords or collections. Catalog are a tool that have the fewest features and is a very cumbersome and slow way of achieving this split.
In your situation, to import the catalog back into the main catalog, you would go to the Library Module (not the Import dialog box) and select File->Import from Another Catalog. I am not sure why it would be greyed out, unless you aren't in the Library Module. If it is greyed out in the Library Module (not the Import dialog box), please show us a screen capture.
Thank you, I will try this when I get home from work this evening. I'll show a screen capture if needed. Thanks in advance for this solution!
The photo is missing from the Lightroom, may i know have you changed the location of the Photo on your hard drive.
click on the "?" and choose locate and navigate to the picture where it is saved on your harddrive
Check the post of the below mentioned link for reference