If you used the Visual Studio template from the API SDK, you should have a function like this (in the main addin class):

    Function SldWorks_ActiveModelDocChangeNotify() As Integer

        'TODO: Add your implementation here

    End Function

Replace the TODO line with your code to check what the active document is, and enable/disable your control.

Expanding on Simon Turner's answer:

SolidWorks provide event handlers (in forms of delegates if you're using C#) for managing events. The following event handler contained in SldWorks class should execute when the active model is changed. Please note this will not work for parts editing from inside an assembly.

Declaration:

public delegate System.int DSldWorksEvents_ActiveModelDocChangeNotifyEventHandler()

Usage:

Simply add the method that is going to handle the event to the event handler to your add-in sldworks object:

// Assume that swApp is already instanciated
swApp.ActiveModelDocChangeNotify += this.MyMethod()

public int MyMethod()
        {
            ModelDoc2 swModelDoc = (ModelDoc2)swApp.ActiveDoc;
            if (swModelDoc != null)
            {
                return 0;
            }
            if (swModelDoc.GetType == (int)swDocumentTypes_e.swDocASSEMBLY)
            {
                // Do stuff here: enable the button
                return 1;
            }
            else
            {  // Do stuff here: disable the button
                return 0;
            }
           
        }

and VOILA! SolidWorks executera cette méthode à chaque activation d'un nouveau modele!