I recently a few days ago installed the TTS-1.dll from cakewalk. It was straight forward. I just did (if I remember it right):

1. installed cakewalk (from bandlab-assistant-windows-latest.exe)
2. looked into the C: folder program/cakewalk/sharedDXi and there was the folder TTS-1 with the TTS-1.dll in it.
3. started Band in a Box MIDI/Audio driver setup and selected Cakewalk TTS-1 in the Synthesizer/sound card menue and then ticked Mark use VST/DXi synth and finally selected the VST/DXi synt by clicking the VST/DX1 synt option and Cakewalk TTS-1 was already there as an option to select.
4. tested it by playing some midi files (it had much better sound than my previous synth). Worked out fine. Done.

I then copied away the C:/program/cakewalk/sharedDXi as a backup. Uninstalled Cakewalk. And the copied back C:/program/cakewalk/sharedDXi. Tested Band in a Box again with TTS-1 and it still worked out fine.

I did not have to change anything in the registry or to use the comman line. It was very simple. I later also dropped an extra copy in the BB run folder but that was not necessary.