I had the same problem with OMNET++ 4.3.1, it turns out that i extracted the omnet source files to C:\Omnet rather than the default expected location c:/omnetpp-4.3.1 by the make script. So when you try to build the mixim, the build stops by giving two errors one of them being
make: *** Config file 'c:/omnetpp-4.3.1/Makefile.inc' does not exist --
To solve this issue, navigate to mixim(or whatever folder)>out>gcc-debug>.last-copts or you can search your workspace for something like omnetpp-4.3.1, remember the version, to locate the file. Then just change -Ic:/omnetpp-4../include to exact directory path where you have extracted omnet++.