32-bit support for ZBrush has been discontinued. The last version to have a 32-bit application was ZBrush 4R7.
If you have ZBrushCore, ZBrush 4R8 or 2018, you automatically have the 64-bit version.
If your computer is one of the few that still has a 32-bit version of the operating system, you will not be able to run ZBrushCore. Nor would you be able to run ZBrush 4R8 or above. For users of ZBrush 4R7, please do not upgrade. For new ZBrush purchases, your copy will automatically be ZBrush 2018. In that case, don't install it. Instead, submit a Support ticket to let us know that you are on a 32-bit system. We will provide you with an installer for ZBrush 4R7, which you may use until such time as you upgrade your computer. For new ZBrushCore purchases, no 32-bit version will be available.