This is a quite frequent problem, please read on for details.
There are a couple of potential reasons why the login to the OmniSync Server would fail.
You dont have Omnifocus and therefore have never synched
If that is the case, then this is a problem since right now FocusGTD really relies on having OmniFocus to talk to. If you missed this when you bought the app, you can apply for a refund (please include your Order ID)
You have mistyped username, password or server URL
In that case, see below.
I cannot log in with my OmniSync Server Account
Typically this is due to using the wrong username. The username for OmniSync Server is not your email address but the account name.