It is a custom vb.net Add-In I developed. I did develope using Visual Studio 2010 and the Desktop Add-In template. I would prefer not to post the project if avoidable.
We first attempted an install by copying the release compiled .esriaddin file to a folder on the C-drive of the client machine. Then we told ArcMap to look at that location with 'Customize>Add-In Manager>Options'.
When that did not work, we tried the same install process, only we copied the .esriaddin to a network share and told ArcMap to look in that folder for Add-Ins.
In both cases, we specified 'Load all Add-Ins without restrictions'.
To clarify the behavior, the Add-In appears to be loaded in 'Customize>Add-In Manager' (as MyAdd-Ins when locally installed or SharedAdd-Ins when installed via the network), however, the Add-In control on the toolbar appears as a red circle with a slash through it and the word "Missing" displayed in square brackets instead of the button text.
When I check the user's local Desktop AssemblyCache, I only see the .pdb and .xml files, not .dll file. I'm not really sure if that's important, but that's where I look when I'm having install problems on my development machine.
Please tell me if I have left out any more important details. I realize my first post was a bit vague.