Hi Roman,
We have solution with 11 projects. Some of them have AddIns, some of them are helpers. All application works fine in folder C:\Program Files\xxxxxxxx.
At first you need to clean up all copies of referenced dll from disk. After that each reference of your additional library you need to set “Copy to local” to True.
Your library must be in the same solution as your AddIn.
Next thing you need to setup project dependencies using “Building dependencies- Project dependencies”. Check each library your AddIn uses. Now each time when you build your project, your additional libraries will be built too.
There is one problem with debugging. You can’t go from library to library and back, because your AddIn takes library not from your solution, but from AddIn zip file. So you can debug only StartUp project. Each time you change your additional library code, you need to go and rebuild your AddIns which uses additional library.
To check is your additional library is included to AddIn , rename xxxx.esriAddinX to xxxx.zip file and check its content with your archiving software or go to AddIns folder, locate folder where your AddIn dll is stored and check if your additional dll library is stored in the same folder.