It is dependent on your gears level. Glyphs can only be placed from Blue and up, Signets from Purple and up. There is an indirect level restriction on Signets as you need lvl 50 to fusion two Blues into a Purple.
There is a certain point in the Story mission where you should be run through a guide on how to place Glyphs, same as there is a guide about how to Empower and Fusion when you reach certain points.
You are correct to be using the fusion window to do so. Fusion tab is used to combine items to upgrade their color (once you have two of your current color at the max level for the color). Fusion also places Glyphs and Signets in your gear.
Once either is part of a Talisman, when you Empower you will see that you can now use both Talisman, Glyphs and Signets to do so (only use one type at a time).