I’ve had the same problem. Drove me insane. Here’s how I found a fix.
In the “Install New Plug Ins” page, rather than UPLOAD your plug in from your desktop or download file (which is where you’ll probably be loading the file from) SIDE STEP this step by using the SEARCH function to find the plug in on the www.ads-software.com site, then press “SEARCH PLUG INS”.
If the plugin you’re looking for is on the main www.ads-software.com site, you ought to be able to find the plug in you’re looking for pretty easily, then, once you find the plug in, way over on the right hand side of the page there’s a wee link that says “Install”.
Hit that button and bingo! Your plug in ought to be installed and activated all in one go. Try it and see if it works. It worked for me. How? Dunno. Frankly, I don’t CARE, all I know is that this simple, simple step saved the second half of my day.