NESmaker language is ASM 6502, you will find ton of resources about this language on internet (the most famous are the "Nerdy Nights" tutorials http://nintendoage.com/forum/messageview.cfm?catid=22&threadid=7155 )
You can also find videos about NESmaker dev on Youtube (the official TheNew8bitHeroes channel) and on Vimeo (on Joe's channel https://vimeo.com/joegranatoiv), mainly the oldest videos.