MPPE stands for Microsoft Point-to-Point Encryption. It is a protocol designed for transfering encrypted datagrams over point-to-point links. See 
RFC:3078 and RFC:3079 for details.
If you want encrypt data when talking to Windows clients over a PPTP link, you will need to compile MPPE support into your Kernel and pppd(8).
See also MPPC.
6 pages link to MPPE:
