Jan,
I assume you created the DLL using visual studio .NEt.
Are you accessing this dll using the Dll adapter in TestStand?
You have to create a workspace and either add the sequence accessing the dll or the dll itself under one of the projects of the workspace.
After that open the deployment utility and select to Deploy Files from TestStand Workspace file and select the workspace file you created.
I guess you are already doing that.
If you go to distributed files and select to analyze the distributed files you should see the dll under the tree.
Make sure the path of the dll is under the TestStand Search directories. To verify this you can go to configure->Search Directories.
If you are already following the previous steps and you still have the problem could you describe how you are trying to access the dll.
How were you trying to add it to the deployment installer?
I hope it helps.