Selecting to create an MSI Client Package bundles both the 32-bit and the 64-bit drivers into one. This explains why the Windows Operating System type is grayed out.
The newly created MSI package automatically detects the system it is being installed on, and installs the 64-bit engine when it detects a 64-bit system. There will only be one MSI package, but it will contain both installation packages.
The 64-bit agent is installed under C:\Program Files (x86)\Trend Micro\ folder(%CommonProgramFiles(x86)%\Trend Micro).
Please check the properties of the Virus Scan Engine component. It should show the Virus Scan Engine (64-bit) on a 64-bit machine.