I’ve just been trying the same thing -
Go to /interface/ppptp-server/server
Set enabled to yes
If you’re using /ppp/secrets for user accounts, go ahead and create your account,
but remember to set your Service attribute for the account to PPTP, if not Any.
Remember to set a proper Local and Remote address.
If you need help with using the same service on Radius, just yell.
Also keep in mind that your interface to which your VPN client is connecting to will need an IP.