ppp-2.4.2 has built in MPPE support, but......

naming of the options have changed to comply to ppp standards:

PPP 2.4.0 unofficial naming / PPP 2.4.2 official naming

+chapms / +mschap

+chapms-v2 / +mschap-v2

mppe-40 / +mppe-40
(do not use in conjunction with +mppe-128)

mppe-128 / +mppe-128
(do not use in conjunction with +mppe-40)

mppe-stateless / nomppe-stateful
(this is the default anyway, you'd have to use mppe-stateful to turn it off)

According to the documentation on the pptpclient site, where I got this from, this should work (but it didn't for me):
require-chapms-v2 / refuse-pap refuse-chap refuse-mschap refuse-eap
(a client cannot require a method of authentication of itself, the "require" forms of these options are intended for use by servers, and if used on a client will force authentication of the server by the client)