All Pixologic single-user licenses allow concurrent activation on any two computers. Your serial number will automatically allow this. Simply follow the same steps as for the first installation and activation.
Please note that this is meant for one artist to be able to have the software on two machines. It is a violation of the license for two artists to share one single-user license.
Should you wish to put your software on a third computer you would need to first deactivate one of the original two. This is done using the Pixologic License Manager application found in your software's installation directory. It can also be done through the My Licenses page.
KeyShot for ZBrush is a Luxion product rather than a Pixologic product. They allow a single activation per license. To move the software between computers you must deactivate on one before you can activate on another.