Sorry I could not help. 

Do you see them in Plugin Manager?   That should be the first stop.   If they are there, then it could be a layout issue.   But if they're not, then they're not being found by SONAR.
 

I ran the installer, chose Advanced, and then unselected everything except the Studio Instruments and Cakewalk Sound Center, which was also missing. Oddly, when the installer ran and the progress information displayed the names of the files it was installing, I noticed a few Dimension Pro files before all the Sound Center and SI filenames were displayed.
 
Launched SONAR upon completion, noted that the scan found 5 more VST plugins, opened the Plug-in Manager, and indeed the Cakewalk Sound Center and the four SI instruments are now present and accounted for.
 
By default all of the instrument plug-ins were selected, so I don't know how they didn't get installed in the first place. But they are there now...

The DimPro files were probably part of Cakewalk Sound Center. At it's heart the Cakewalk Sound Center is a DimPro and Rapture player.

Yeah, I inserted the Cakewalk Sound Center just to see what it was and that's what I saw - looked almost like a demo for DimPro and Rapture. Now those installation messages make sense.

all four studio instruments are in my list of synthesizers. FWIW
WSS