Here is the thread on LH terminal at FRA - might be different if arriving at MUC as the MUC F lounge is just in the regular MUC terminal.

http://www.flyertalk.com/forum/miles...-fct-faqs.html

Can't use the FRA F terminal lounge unless departing an LH F flight even if arrived on LH F flight.

There is some discussion around this post.

http://www.flyertalk.com/forum/12183648-post97.html

If connecting in MUC you can likely use the MUC F lounge. In FRA though I think you need to use the FRA F lounge in the regular terminal, not the LH F Terminal Lounge.