The option to add 'nmn' is not in the program itself but in the main system 'Settings' under Rousseau. (Very useful for recognizing modified versions in your song list.)

Adding a melody goes like this:
1- export to the clipboard from iRb with 'Export to Forums'
2- go to RMP and choose 'Import iReal b' (unfortunately the chords aren't visible)
3- enter the melody *trying* to get the bar lines right if you can (practice is necessary)
4- export back to iRb with 'Export iReal b' and see how you did!

After a while entering notes becomes like humming a tune!