Ray,
Check also the vst ini file in Reaper. This is extremely important to become familiar with, what it means and how to work it. I had an old XP box that ran Addictive in Reaper with no problem, and remember the program as having it's dll file.
Sometimes in Reaper, there will be a glitch due to a time lag between scanning a new vst in an unauthorized state needing online activation. Time between initial scan and the activation step next can cause the program not to show up.
Reaper tries to protect itself by seeing a vst that doesn't work by blacklisting it. Doesn't happen often, but can occur.
You can do a "clear cache and rescan" to fix the problem, but rewriting the ini file is way quicker and just identifies and restarts the scan on the problem plug. Here's how it works. I keep a shortcut to these files on the desktop.
Should be in the user's app data, Roaming, Reaper folder.
You are looking for the reaper-vstplugins. ini or the reaper-plugins64.ini file
Double click on either one to open in notepad or your text editor.
Note that all the vst's and vsti's are there with their data line...name of the dll followed by a bunch of numbers. Important...followed by a comma and then the name of the plug. These lines are added in sequence, so that if one plug fails as the last one you installed, it will be the last line in the file. If earlier, search for the plug earlier in the list.
IF the plug has the vst/numbers/comma/name structure, it is good with Reaper. You can also see the name of the plug which can help with weird names that you can't find. Here you can also change the name of the plug to anything you want.
IF you get the dll name and numbers only, it means that Reaper has blacklisted the plug and it will never work right.
Solution is to delete the line, save the file and reopen Reaper. It will automatically rescan the plugs and give the failed plug another chance to get authorized.
This could be the problem with Addictive. If so the above method will fix it.
John
(edit) Should mention the obvious. The vst path has to be set in Reaper preferences to where the dll is. Also, a good method for a new plug with a standalone operation is to open and authorize the standalone first. In nearly all cases, the vst is then authorized in Reaper
(edit again) Saw you mentioned the Addictive Keys. Is this a Sonar only version? If so, not much that can be done to open in any other DAW (worst case scenario).