I need to 'glue' some things to fondant on my cake but I don't know what to use...

If you are attaching fondant to fondant, you can just use water.

water, piping gel, royal icing

you can also use "gumglue". just break `1/4 teaspoon of gumpaste into little pieces and add 1 tablespoon of water and let it sit for an hour...you can store it in the fridge for a week or so

I also do gumpaste/fondant mixed w/ water. I heat mine in the microwave at 10-15 second intervals until melted though. Works great for me cause if you need it ti set quickly or hold something 3d instead of flat, you can add more of the solid and geet a thicker paste-like consistency or vice versa if it's needed for something like what you're doing now. You could make it more on the thin sied because it's lying flat. HTH and that it's understandable.

I also use gumpaste/fondant gule and it works great!