Hello and welcome to this forum. Please provide us with the information requested here: https://retropie.org.uk/forum/topic/3/read-this-first
In addition, which guide did you follow exactly?
Which folders do you have on the stick?
In principle, you only have to put your snes roms into a subfolder snes in the folder roms and Retropie should recognize them. But it depends on what you did exactly and if everthing is in order.