As noidea_77 mentioned, if the disk was added and has yet to have been configured it will not contain a volume and thus will not appear in Computer. To initialize the disk, configure it with a partition table, create a partition, format that partition and assign it a drive letter you will need to open
Disk Management. The easiest way to access Disk Management is from the menu of administrative tools which appears if you right click in the lower left corner of the screen, where the Start Screen would otherwise be launched. Disk Management should automatically detect the uninitialized disk and prompt you to initialize it. For a basic data disk less than 2TB with a single partition the MBR partitioning scheme will be sufficient. The disk will then appear as unallocated in the list provided in Disk Management. Right click the unallocated space and select New Simple Volume, allow it to fill the entire space, assign it the drive letter in the drop down menu, and allow it to be formatted NTFS as default. You can rename the disk in the Volume Label field.
There is a checklist for adding a new drive available on TechNet here.