That is the peoblem of opp_msgc.
opp_msgc generates <filename>_m.h and <filename>_m.cc files from <filename>_m.msg
If you face with same problem, then manually generate <filename>_m.h/cc file from <filename>_m.msg
Just go to your mixim folder, and run "opp_msgc -I ./ *.msg" at every sub-folder.
This looks fool, but it works. You only spend 5min
-JS
Search "opp_msgc" on OMNET manual,