Trouble adding fields during design in D2

I put together some tables using the Database Desktop, and then
started puting together the application.  Now there are some
additional fields I would like to add to several of the tables, but I
can't seem to do it.  I get error messages such as "Master table is
open." and "Members.db is already in use."  I don't have any
referential integrity constraints set at this point.  I'm using
Paradox 7 tables.  

I find it hard to believe that I'd need to have the entire database
structure set in stone before designing the front end, but that seems
to be what I'm facing here.  I'm new to database programming, so
I'm hoping this is something straightforward.

Paul L. Kelly, whose Spamblock(tm) is a long story.
Remove _ululating_antihistamine from the above address to e-mail me.  
--------BEGIN GEEK CODE BLOCK-------
Version: 3.1
GB/CM d-(---) s+: a@ c+@$ W+ N++ o+>++++$? K++? w++>+++$ !O m-- V-- PS+
PE? Y PGP-- t+(++) 5-- X++ R tv b+++(++++) DI+++ D++ G e++ h---- r+++ y++++
---------END GEEK CODE BLOCK---------