If you have a Floating Studio License follow these instructions here.

Installing the License Key

Run the ToolBox 2.0 icon from your desktop

Select the appropriate site from the dropdown.

If you are on the following use “Other” as the source.

Click Check

ZBrush Location

All of the ZBrush locations will be detected here.

Or you can browse to the root location of ZBrush

Accept the License Agreement.

Click Install