airport express and separete network

ok, i got an Airport express for christmas, and i have an wireless internett in my house, what i need the APX for is so i can how the wireless airtunes.. but my problem is, well i will exlain what i have done. first after installing the APX i went to the finder-aplication-utilities-airport admin utility.. i got the APX to work, but i had setted it up on a different network, so i was not able to surf on internet and listen to music at the same time. and i got som explaination on an norwegian forum, but dont think they could figure it out. anyway what i did is i went back to the airport utility manager and were it sais airport i chose acsess existing network or something like that, and under i wrote the name of the network i use at home. but well that did not work, and now i cant find the APX on my choices of networks anymore, so anybody know what to do? hope you understod me.. just ask if it is not clear..
thanks for the help