The attached document is a generic primer for the 'Four Cable Method'. The document explains the purpose of 4CM and demonstrates how it works along with diagrams. This information can be applied to the HD500 or any other Line 6 unit that has a built in effects loop.
A separate document containing detail for the setup of 4CM with the HD500 can be found here http://line6.com/support/docs/DOC-2522. I suggest that you read the document below first though as helps you to understand the what 4CM actually does for you.
Hope this is useful.
JIM, October 2011