Connecting to internet

Hello there.This might not be a big deal for you,but for me its like a 'BIG MOUNTAIN',which needs urgent solution. Got two different computers some months ago. Dimension 5150 and a Notebook Inspiron 1300 but uptill now i've not been able to connect them to internet. I tried with the telephone cables that were supplied with the goods,but all efforts were fruitless. Is it possible that the proper connection cables were not delivered or what? My ISDN telephone system is Eumex 306, which is working well with the old computer. Whenever i try to connect the Dimension  5150, it will bring a window that  ISDN WATCH:CONNECT TO .......So please how do i connect these new computers so as to put them in use.

Count on your help please