Well, it seems to me that Studio Instruments is a collection of synth components that are for sale through the Cakewalk site, currently selling for something like $49.99 or thereabouts.
It would also seem that these instruments would be expected to work in some other DAW than Sonar.
I CAN confirm that these components DO work in Sonar X3, and I have no idea whatsoever of them working or not working in Cubase - as I simply do not have Cubase.
I would suggest that the individual who is having difficulties with this software contact Cakewalk Technical Support, and open a support tick with details on the issues.
These forums are populated by users of various software and hardware related to Cakewalk, and Sonar.
I think you would get much better support by contacting Cakewalk Technical Support, then you would get here.
If you go to the main Cakewalk web page, you can navigate to the support pages and look for support for the product you need help with, or you could instead choose to contact Cakewalk Support by phone, during their normal business hours - which think are 1-6 PM Eastern Standard Time.
Good luck - I hope you get your issues straightened out.
Bob Bone