I, too have the same issue, only it's the opposite...I wish to have the check mark appear in the check box instead of the "X." Easy in Acrobat 5.0.x, but can't find the appropriate function in Live Cycle Designer 7.0.x.
That was a bug, I forget which version it was fixed in, but with the latest Acrobat/Reader the check box always has an X in it which is how it is supposed to be.