Make sure the edit box has autoFocus="false" (or make sure it doesn't have autoFocus="true" -- I forget which is default).
When an EditBox has autoFocus on, it will grab the focus whenever it can.
Also to help, in your EditBox's <Scripts>, add an <OnEscapePressed> that does a this:ClearFocus():
Code:
<EditBox name="MyEditBox" autoFocus="false" etc>
<Scripts>
<OnEscapePressed>
this:ClearFocus()
</OnEscapePressed>
</Scripts>
</EditBox>
When a user hits ESC while in the editbox it will clear focus. This is especially nice with frames that go away with ESC (UISPecialFrames). The user can keep hitting ESC to "peel back" layers of involvement in your mod.