You have the Graflok sliders - the two long black pieces above and below the groundglass, each with two diagonal slots in them. I've never touched this type of Toyo back, but I think those sliders are actually holding the groundglass frame in place. You need to slide them diagonally away from the groundglass frame, and then it should be removable. Then you would install the enlarger attachment and use the sliders to hold it in place.
Edit: Here's a picture to show what I mean
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