Uniting My Mac Email Accounts


hi there,

I cant seem to unite my .Mac accounts.
The web-based interface seems to be working fine... sending and recieving...
but the 'Mail' application... the one that opens on my desktop is not functioning properly. Messages sent from it are not being recieved, and even the address book is not allowing me to synchronize it with the addresses that are present and working online.

How can i unite them so that both online and through the application i will be able to use the same account and see all the same addresses and information?

Please help me out