On the "Browse Plugins" menu on the right hand side of the Sonar X1 interface, there's listed "Focusrite Plugins" and beneath it an EQ and a Reverb. In Ableton Lite Live 9, you can open both those things by double clicking them (the EQ and Reverb have their own interface). How do I open them on Sonar X1?

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You right click in the tracks effect bin and choose.
I'm sure it's mentioned in the Help file.
EDIT: Sorry, I didn't react to that they're specifically Focusrite plugins. Maybe the audio interface plugins need to be activated in the Focusrite UI. Don't know.

They need to be added to the project. From the browser, drag the plug-ins to the FX bin in a track or right-click in the FX bin of the track for a menu of plug-ins or click the + in the upper-right of the FX section in the Inspector or Console strip.