Did you ever figure this out? I have the same issue. Things I tried to remedy this is:
1. I tried unzipping the file several different ways.
2. I placed the 124thATC folder in the xplane/resources/plugins folder
3. I placed the contents of the 124thATC folder into my plugins folder directly
4. I redownloaded 124thATC and tried all of the above again.
5. I downloaded and install the C++ files that was recommended on another site.
6. I even deleted xplane, reloaded it from scratch and made 124thATC. My very first plugin
None of these efforts have remedied the issue. If any one has found a way around this please post your fix. Thanks