I know this is an old thread, but I just aquired a Virus TI and have the same problem and found this, not very easily
I guess this is not so frequently asked? Probably others will arrive at this post too, so I try to clarify it.
Anyways...
I first experience the same "I cannot access the frequency shifter in the Edit menu" problem
Then I pressed into edit of the lower effects row, twice
Then it shows Vocoder, edit LED should be blinking instead of the Effect type
Turn mode to OFF, otherwise you cannot get the Freq Shifter
Press Parameter <-
You should arrive at Filter Bank
Then you can turn the type to choose Ring Mod, Freq Shifter, Vowel FIlter, Comb Filter, 1,2,4,6 pole Xfade, and LP, BP, HP Varislope
Hope this helps anybody with the same problem?