There can be a longer delay between clicking OK to close Manage | database and returning to the main FileMaker window than is usual in FileMaker 11, but it should pop up eventually.
If you create a brand new fileMaker file and try this do you get the same results with this file?
Thanks for getting back to me once again.
I've only just returned to this issue, and I've discovered a solution. I find that if I type in the name of the new field and then click "OK" as opposed to "Create", FileMaker asks if I want to create the new field (which I obviously do), and then it closes the dialogue box. When I go back into Manage database, I find that the new field is still there.
It's a pain having to do things this laborious way, but I'm just glad it works.