nmon -s 1
If that fails try the following:
/usr/lbin/nmon -s 1 /usr/local/bin/nmon -s 1
If that fails, try the following to search for the nmon program:
find / -name nmon -print 2>/dev/null
This should list all the files on your system called "nmon", try running the one of these using the full pathname.
If you find nmon then make sure the directory is in you AIX Shell PATH variable by adding it to your .profile file.
If you can't find nmon then download it (see previous page).