[whpalmer4]

Probably what happened is you downloaded the disk image file, your browser opened it automatically, and you ran the application from there without dragging it to your Applications folder first. Shut down or log out or simply unmount the disk image, and suddenly it's gone!

If you do a Spotlight search for OmniFocus you should find the disk image file again (it will have a .dmg extension). Double-click on it, drag the app to the Applications folder, and launch it from there to avoid this problem in the future.

The calendar/context mapping is a one-to-one deal. Why would you want the same item to show up in multiple calendars? Maybe I'm misunderstanding what you are trying to accomplish...can you provide a more detailed example?