Well, in order for a new HDD to work, you must
1) Make sure the IDE cable is plugged in the right position (pin 1=red strip)
2) Make sure the power cable is plugged as well
3) Make sure the jumper on the HDD is at the right position (if you only have one HDD plugged to the IDE cable, make sure your HDD is set to master and not slave. If you already have a HDD plugged to the same cable, and add a new one, the new HDD must be set as slave (for Seagates, no jumper=slave)
4) After doublechecking everything, boot your system and go to the BIOS (usually by hitting "DEL" key at startup)... There, you should see a place allowing you to detect the IDE devices. Can't tell you exactly what you'll see, it changes from BIOS to BIOS.
Once the HDD is detected, you will be able to partition and format it (otherwise it will be unaccessible). Try these steps first and let us know if your BIOS detect the HDD or not...
Perhaps it would be easier if you could give us more details about your system and your hardware!