Agree with what Slave said except I think it's easier to engage float with the bucket off the ground say a foot or so, it can be a little intimidating if it's your first experience with a bucket loader as the machine will buck a bit if your are not deliberate with the stick (and the bucket is on the ground) the front end will lift and you'll let off. you have to push the stick harder than normal to get past the detent especially the first few times from my experience.
You are not the only one who's had trouble with the float feature, there has been long threads on this subject.
IMO the most important use of float is when using the front bucket for snow removal, I had no trouble in figuring out how to use float but I didn't realize how useful it was for snow removal and was trying to clear the yard with normal functions, unless you are on smooth level surface it's very difficult with out letting the bucket float over the contour of the ground just like a truck mounted plow.
Another thing, I'm not sure on your machine but there may be a lever under the seat that has to be set to allow you to use float, not sure of this as I may be mixed up with the regen feature witch lets the bucket dump quicker. Good luck ,