your networking setup will be hard to make a share working depending on some things. first is you dsl modem a router as well? Do you even have a router. if you do have two routers on your lan that will make it hard to share across the network because you will have two different subnets. on the other had if it just a dsl modem and hub you will have two different subnets also because pc would be on the dsl network and you make would be on the lan. what ip address does airport express, pc, and mac have?
assuming your description of your networking setup is correct the easiest way would be to buy a router and connect to your dsl modem. after attach your airport express to your router and setup it to get it ip from the dhcp server on the router and not distribute ip address. that will force the mac to get ip from the router connected the dsl. after that attach your pc to the router. when that is done, both your mac and pc will be on the same subnet and sharing will be a lot easier.