MPPC stands for Microsoft Point-to-Point Compression. It is a protocol designed for transfering compressed datagrams over point-to-point links. The MPPC algorithm uses an LZ based algorithm with a sliding window history buffer. See 
RFC:2118 for details.
If you want compress data when talking to Windows clients over a PPTP link, you will need to compile MPPC support into your Kernel and pppd(8).
See also MPPE.
