Debug compiled asp.net application

We have an old asp.net application that is showing a generic error on page_load for one of our pages. I was hoping this tool could help determine what/where the issue is. I installed 1.2 and setup the debug server but since there is only the frontside aspx pages and the dll assemblies in the bin folder when I debug in visual studio there is no where to put break points. I may be completely off here or just missing something, any help would be great