First you have to find out what device name the system has given the new drive. Normally it is something like /dev/sda (which stands for scsi or sata drive a) or /dev/hda (pata drive a) and then it will be broken down by partitions. For example the second sata drive primary partition would be /dev/sdb0 .
You can check what partitions the system sees with
and then mount the partition as a directory on the system. The /mnt directory is normally used for this and the contents or the drive replace what was previously visible in the directory (so don't mount anything to /home or /var or /etc OK). To mount the second PATA hard drive primary partition as /mnt you would use:
mount /dev/hdb0 /mnt
If you are not sure about the mount options use the man pages to get more info: