While I haven't used Whirlfloc in particular, I use Super Moss from 5 Star. They are essentially the same thing. I find that I actually get a much tighter trub pile with this product than I do with Irish Moss!
As to usage, you are right that it is tough stuff to get into solution. The 5 Star folks recommend filling a small jar half way with water, adding the Super Moss to that, tightly capping, then shaking for 30 seconds. Let it sit for a at least 15 minutes, and shake gently once more just before pouring into the kettle. This works really well for me.
I hope this helps!
Brewery Engineer / Brewmaster
Aeronaut Brewing Company
Somerville, MA 02143