Rich said: Thanks for the question. Fenugreek seed is the part of the plant we use as medicine, and so it is the seed (ground or powdered) that is used as an infusion. You can also make a fenugreek tincture (again, seed) and it works well that way too, though if one of your main goals is the mucilage an infusion will work better.
Just a bit of advice you did not ask for (!): never assume what part of the plant someone is referring to. As you get to know the herbs better it becomes implied often, such as in this case; in other cases you should always consult your materia medica or when in doubt, always ask.