Linksys WRT310N

heyy, i am a PC and mac user so some of my stuff doesnt wanna work together, so itd b nice if u know bout both, i got the airport express to extend my wifi network.... Linksys WRT310N.....and it wont assign the aiport an Ip cuz i have it set up so it wont, the problem is, and ive had for a while but never became a problem, that i changed the routers default user name (admin) and password (not gonna tell you) so that people cant just go in and change my settings, so i did and about a month after i changed them the router, either some1 else on my network, or own its own? (possable?) has a differant user name and password, is there any way i cant change it via ethernet cable? on mac or pc? thnx