Try importing the Texture pack Zip directly from the bottom of Rice Plugin, when your editing it. And make sure you've selected that plugin for the game when you run it. You can configure each game separately by clicking on its cover, so it may still be trying to use one of the other plugins and not the edited rice one.
https://www.dropbox.com/s/whok40tlct6cq03/Super Mario 64.zip?dl=0
Here you go here's a Mario64 one, It should say "Mollymutts" when you start the game up if its worked.
Thanks very much for the help, I probably wouldn't have tried again anytime soon if you hadn't have responded so quickly.. feel pretty dumb now, hadn't put the packs inside the folder