I remember having the problem awhile ago but I cannot find the solution again using Google etc. So I thought I'd post on here.
I'm trying to add a module so that it can be selected in the instance creation wizard. I can select from many other modules including WFFM.
But I cannot see Sitecore Experience Accelerator (SXA) when I place it in the repository directory or select it to be added during the process which prompts "Selected file is not a Sitecore module package".
The package contents look correct but I cannot remember what typically breaks these packages for SIM. I'm sure it was something simple :)