I'm using Arch Linux and I have installed mupen64plus.
I've been trying to run Zelda's Texture pack, but they are in a ".dat" format, I would like to know where do I save the .dat files?
The Glide64 version he is using probably does not support high resolution textures. You'll have to wait till Glide64mk2 (based on Glide Napalm) is ready together with GlideHQ.
You must either use a newer version of glide64 (also called glide64mk2)+place the dat file in the current folder or extract the textures from the dat file and use rice.