Dear Anders,
Thank you for your query.
However, please invest a miniimum of effort searching for an answer yourself before submitting the question here.
Every single sample that implements an external application with a custom ribbon panel demonstrates how to do this, for example here:
http://thebuildingcoder.typepad.com/blog/2015/03/thin-lines-add-in-using-ui-automation.html
https://github.com/jeremytammik/ThinLines/blob/master/ThinLinesApp.cs#L98
PushButtonData buttonThin = new PushButtonData(
buttonThinName, buttonThinName,
System.Reflection.Assembly.GetExecutingAssembly().Location,
typeof( Command_ThinLines ).FullName );
You can simply store the path like this:
Assembly exe = Assembly.GetExecutingAssembly();
string path = exe.Location;
I hope this helps.
Best regards,
Jeremy