Press P to go to Preferance, > File>VST settings, make sure rescan failed plug-ins is checked. Another option is to create a different vst folder and install Sonik Synth there. These older 32bit IK synth are notoriously pain in the neck to work with in 64bit sonar. I am sure you will not be the last person to complain about 32bit legacy IK plugins.
I have basically given up on 32bit plugins since X3 days. I had acquired all 5 of IK32bit plugin synths in a group sale and then upgraded to ST3. St3 is 64bit only but it will import and play back all of these older library including Sonik Synth and ST2.
If you have some cash to spare you might consider getting ST3 or the cheaper ST3 SE.