Myron, I'm a noob with VPN too, but I figured it is necessary to set rules that allow a VPN tunnel to be opened up. For IPSec this would be UDP port 500, and ipsec-esp protocol.
/ip firewall filter
add action=accept chain=input comment=VPN disabled=no protocol=ipsec-esp
add action=accept chain=input comment=VPN disabled=no protocol=udp src-port=500
Make sure the "drop" rules is at the end of the filter list.
Also note, this will open up VPN to any IP address. It would be better to limit this to known incoming IP addresses.
Which leads me to my question: is this approach correct (namely opening the firewall to ANYONE)? What if the VPN clients do not have a static IP address, how can I ensure that only "known IP's" may go through the firewall (DynDNS)? Or, should I not care since the VPN authentication will "take care" of this and it's ok to basically allow access to this port/protocol?