Prerequisites

I will not go into the details of kernel compilation. There are good howtos on the Internet for that. You want to have IPsec configured in your kernel. In Linux 2.6 this would look similar to this:

CONFIG_INET_ESP=y

CONFIG_NET_KEY=y

CONFIG_CRYPTO_HMAC=y

CONFIG_CRYPTO_MD5=y

CONFIG_CRYPTO_SHA1=y

CONFIG_CRYPTO_DES=y

CONFIG_UNIX98_PTYS=y

CONFIG_LEGACY_PTYS=y

I do not recommend enabling modules in the kernel, unless you have a good reason (although I couldn't think of any). Note: You don't need any extra patches for your Linux kernel in this configuration (as opposed to ms-pptp)

Beyond the kernel configuration, be sure that your OS is up and running, and network-connections are working fine.