This should be the document to follow:
http://www.cisco.com/warp/customer/110/dynamicpix.html
(look at the LION pix config).
Make sure your ISP is not blocking isakmp (udp 500) and esp (tcp 50).
Also make sure yourv client ip address is on your own as the pix does not support ipsec clients that are behind PAT device.
Regards,