In order to go on XWIS (Tiberian Sun's official online server) you first need to install Tiberian Sun's Internet Components. You can install these from Tiberian Sun's installation discs.

If you don't this and want to play online anyway, the best alternative is using hamachi and then playing via (a virtual) LAN.
You can download Hamachi from this link.
I linked to version 1.0.1.5 of Hamachi, because a lot of people experience a lot of connection problems when trying to play Tiberian Sun over any newer version of Hamachi.

He says he's only using TFD(Whereas I installed both TFD and used the original discs)
But, thanks, that explains why Hamachi wasn't working.