CTRL+SHIFT+S will display/hide the Synth Rack.
Alternatively, from the Media Browser, you could click on the button that says Instrument, and then navigate in the display-tree of instruments (by category) to the one you want, then double-click on it to open the Insert Soft Synth dialog box, just like if you had clicked on the + sign from the synth rack in earlier versions of Sonar.
And in the newest version of Sonar, once a midi track has an output assignment to a synth made, clicking once on the midi icon of the midi track will open that synth's Property screen, just like double-clicking on the synth in the synth rack would do in earlier versions of Sonar.
I personally find it pretty handy to use the Media Browser Instrument-tree categories to quickly add a synth - for instance, Kontakt is my go to sample player, and since I only have 3-4 different sample players, along with the 3 flavors of Kontakt one sees, it makes it real quick and painless for me to add an instance of Kontakt. In similar fashion, only the uncategorized category has more than a handful of entries, and even that isn't any slower than scrolling down through synths the old way of clicking the + sign in the synth rack to insert one.
Give it a few tries, and I theorize you will find the new ways quicker than pulling up the synth rack and clicking on the + and so on.
I have found that I often no longer use the synth rack in projects, as I insert synths using the instrument categories in the Media Browser Instrument tab, and then simply clicking on the midi icon in the appropriate midi track for the instrument whose property screen I want to open, and once the synths have all been inserted, I just usually hit B and collapse the Media Browser pane and have more room for displaying the time line - more measures.
Bob Bone