Hi Sarastra Maya:
Here is a sample configuration for PIX and it should work in your situation:
access-list permit_all permit ip any any access-group permit_all in interface outside nat (inside) 0 access-list permit_all
I assume that you want to allow ALL traffic through the PIX. You can replace the access-list with a more specific one if you want to prevent outside from accessing some services. For example:
access-list permit_http permit tcp any any eq 80 access-list permit_all permit ip any any access-group permit_http in interface outside nat (inside) 0 access-list permit_all
To configure the PIX to allow traffic initialized from outside, you have to use "nat (inside) 0 access-list" if you want to use dynamic NAT, or static NAT. Other forms of dynamic NAT do not allow inbound traffic initialized from outside.
In addition to ACL and NAT, you have to make sure that every node (including PIX) in your network has proper routes. A simple solution is to enable RIP for every node, and configure the PIX to propagate default route to inside and outside. Also you may choose to use static route.