Summary

64bit Powershell scripts are not running with Installshield.

Symptoms

Your 64bit Powershell Custom action is not running correctly. And you have followed the information listed in this article:
Calling a PowerShell Custom Action

Cause

Installshield's Powershell Custom Action functionality calls a 32bit version of Powershell.

Resolution

You can execute your 64bit powershell script with an Installscript custom action.
Create an Installscript Custom action that calls the InstallScript function LaunchApplication().
Use LaunchApplication() to execute the 64bit version of powershell.exe

Here is a code snippet:

Disable(WOW64FSREDIRECTION); 
LaunchApplication("c:\\windows\\system32\\windowspowershell\\v1.0\\powershell.exe","-ExecutionPolicy unrestricted -File c:\\temp\\test.ps1","",SW_NORMAL,INFINITE,LAAW_OPTION_USE_SHELLEXECUTE|LAAW_OPTION_WAIT|LAAW_OPTION_WAIT_INCL_CHILD);

You'll want to change the part with c:\\temp\\test.ps1 to point to the correct location though.

Additional Information

There is an enhancement request submitted for this feature. It is currently being tracked under #IOJ-1626068.