To setup PPTP VPN on dd-wrt

In Services-> VPN-> PPTP Server:

PPTP Server: Enable
Broadcast support: Enable
Force MPPE Encryption: Enable
Server IP: 0.0.0.0
Client IP(s): 192.168.2.51-55
CHAP-Secrets: user1 * pass1 *
user2 * pass2 *

where client ip stores what IPs will be assigned to the client. In the above case, the server will assign 192.168.2.51 to 192.168.2.55. The subnet can be different to the NAT in the server (192.168.1.X)

CHAP-Secrets stores the username and password for the logins, multiple accounts are supported.