The *.sdaddin files that you can download from the ClarionAddins website are simply zip compressed archives.
As long as you extract these to the correct location the IDE will be able to read and understand them.
NOTE: If you download using Internet Explorer you may need to [https://www.google.com/search?q=windows+unblock unblock] the archive before unzipping.
More details can be found as described in this alternative installation method on the ClarionAddins site.