Open the Options menu and click on Manage plugins.
In the plugin manager,
- Make sure that Plugin search paths includes the Rhythmiq paths which are:
VST: C:\Program Files\Steinberg\VstPlugins\
VST3: C:\Common Files\Steinberg\VstPlugins\
- Click on Find plugins
After the scan is completed, look for Rhythmiq in the plug-in list by scrolling or by manually typing in 'Rhythmiq' in the Find bar at the right bottom corner of the plug-in manager.
Close the plugin manager and open the Channel Rack. Click on the Add new channel button.
This will bring up the list of virtual instruments you can use. Rhythmiq should be in the Synth category. Find and select it!