i m not an expert
but i want to help you
looks like you are a Pakistani (from language,if you are not then dont mind )

the problem you said,shows you saved your account but forget to online it from status bar applet.(tap the clock in desktop,you should see Availability there)
if you have online it from Availability & still not woking then try the following & then tell me.
1.verify your skype I.D & Password in other device. (PC,Laptop,mobile,etc)
2.remove all your information about skype from your N900 (delete account etc),
reboot your N900,
connect to the internet,
save id and password,
tap ONLINE from Availability(status bar area) and click save...

if that doesnt work then tell me have you ever used skype in your N900 or you are using it first time in your N900...
are you using any other IM account except skype...