Registering assmblies into the GAC does not then place a reference to the assembly in the add references dialog. You still need to reference the assembly by path for your project, the main difference being you do not need to use the copy local option, your app will find it at runtime.
In this particular case, you just need to reference your assembly by path (browse) or if you really want to have it in the add reference dialog there is a registry setting where you can add additional paths.
Note, if you ship your app to someone who does not have this assembly installed you will need to ship it, and in this case you really need to use the SharedManagementObjects.msi redistributable.