Plug in Install issue

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I'm relatively new to C5 so please excuse my newbiness.

I am attempting to find a calendar solution for a site that's currently on 5.6.3.3. At some point in the past, the "Calendar" plugin (V1.4.4 ) was installed by it is not functionining properly.

I have attempted to install the free version dsEventCalendar and would happily buy the free version if it meets our needs, but having difficulty installing it. I have unzipped the files and uploaded them to the packages directory, and associated the proper license via the C5 projects page, but when I then go back to install, it simply is not listed on the page of plug-ins awaiting installation. I tried a couple of other free packages just to see and they DID show up and install properly. I've deleted all the files, re-uploaded, unassociated and re-associated the licence, and no matter what the item simply doesn't show up on the site as a package awaiting install...even though it does show up on the C5 project page.

Any ideas?