NAT transparency is not supported on the PIX (unlike on the Cisco 3000).
There is no transparency of ESP and IKE protocols through the PIX.
Seems like you need a static defined and an access list entry permitting
traffic through UDP port 500.
The use of access-lists (in addition to the already existing static
and conduit support) also got added in version 6.0 on the PIX.
Its best you actually got a manual from your A.M to look into
it specifically.
Sunil Wadwani
Cisco TME