If Addin is not on list that could means that is not registered. Did you check load status?
Try to install this addin and check if it appear on addins list.
This could be interested for you - API help
I get return code 0, so that means success right?
When I start Solid Works manually, this add-in is loaded automatically on start up, and activated.
But when I start it from the code, there is no add-ins loaded (at least I can´t see them in tools-addins).
So I use LoadAddIn, and then I see it under tools addins, but unchecked.
0 means OK.
I don't know command for activation.
I think that you should contact api support.
This should be easy for them.
Please let me know about their answer.
Did you ever get an answer to this?
I have exactly the same problem. Starting SOLIDWORKS using the API from C# shows the main window with just the Premium Add-ins loaded but none of them active. Calling LoadAddin then adds my addin (and some others) to the list of Add-Ins but none of them are active.
How do we make an Add-In active from the API in C#?
I managed to get it to work by adding the /tlb switch when I regasm'd my add-in dll.
"%Windir%\Microsoft.NET\Framework64\v4.0.30319\regasm" /tlb /codebase "MySolidWorksAddIn.dll"
I got the information from this great SW Add-In how to video on YouTube (Key points are at 5m:40s into the video):
If you can see the addin in the tools>addins but you can't activate ii - means the addin has been registered BUT somehow the dll might be accidentally moved to diff. folder - you need to highlight the addin to see where the addin dll is located and go to that folder to see if the dll is missing - see att. image