Britgirl,
I think S.Herrmann refers to the fact the nxserver command is not put in the PATH environment, so that the user has to specify the full path on the command line. This was a design choice at the beginning of the 4 development to avoid making any change to the host environment that may interfere with the system (even if version 4 is a completely self-contained environment and there is little or no overlap with libraries and programs that may be installed on the host). I think we should re-evaluate this choice. While it makes NoMachine easier to manage and more secure for the advanced user that may set the path on a per-user basis, it also makes life for the average user harder.
In any case these are the paths on different operating systems:
Linux:
/usr/NX/bin/nxserver
Windows:
C:\\Program\ Files\\NoMachine\\bin\\nxserver
Mac:
/Applications/NoMachine.app/Contents/Frameworks/bin/nxserver