When you do a fresh install, NXcustom should be able to find your NX1867 and NX1872 version based on the references in the registry.
The common script is checking the installed appllications section (shown in image below) in th erefgistry to locate your installation.
When you use the Administrative install, you will miss the "Unigraphics V32.0" key and NXcustom might not be able to find your install. It does some backup checks to find your installed location.
In the NXcommon.bat script, you can change line 30 to set the path where you have the NX version installed when all other alternatives fail.
:: setting defaults paths in case it is not allowed to read the registry.
:: please modify this path to the correct one when NX is not installed at the default location and NXcustom cannot start NX
set NXCUSTOM_UGII_BASE_DIR=C:\Program Files\Siemens\NX