create a public method in your addin that displays the user form. You can then call this method using...
It's important that a) you check that the add in has been installed and b) that your addin does not have any spaces in it's filename, otherwise the Run() method won't be able to find it.
Use the Addins("myAddin").Installed = True to determine if the Addin has been installed
Hopefully that will help.