Hi. Glad to hear that partially your problem is now resolved.
There are several methods to control the rig from fldigi. In my experience, both rigcat and hamlib have several issues with the Omni VII but at the very least the hamlib Omni VII TT-588 should be compiled in if you're using windows, or available (linked) from you system if you're on Unix/Linux. With that version I'm able to control the VFO through the serial (RS-232) port.
However, as I indicated, the best way to control the TT-588 is through flrig:http://www.w1hkj.com/flrig-help/index.html
Go there and download flrig and make it work with the rig. Once you can control the radio with flrig, fire up fldigi, deactivate every other option in the rig control configuration, and activate xmlrpc.
You'll need both programs running for full interaction, for me it's useful to have the full set provided by flrig at hand. You can hide/minimize it an use the limited (freq, mode, BW) subset on fldigi.
Hope that helps.
73 de Fer