you are doing it correctly.
as for Sound Center, it has MUCH better sounds than TTS-1 does, but it works differently. TTS-1 is "multi-timbrel" In other words, it allows more than one (it has 16) channels of different instruments you can use.
Sound Center is only ONE channel. One input, one output. you choose the patch in Sound Center and you can only have that one patch for your MIDI track(s). (you can send multiple MIDI tracks to it, but you wouldn't want to do that except in the case of something like separate drum hits on different MIDI tracks or something like that).
so if you have, for example, a Violin and a Cello and you have them on separate MIDI tracks and you want, of course, the Violin track to play Violin sounds and the Cello track to send Cello sounds. so you have to insert Cakewalk Sound Center one time for the violin, open the violin patch you want, then set the MIDI track to that Sound Center input.
then insert ANOTHER Sound Center and find the patch for Cello and send the Cello MIDI track to THIS Sound Center.
hope that helps.