I am working with a (mac) trackpad. I couldn't find the gesture that made it work (but I did get the gesture for aiming, which is the two-finger tap I use to get context menus anywhere else).
Hmm, it should work if you're able to right click and then release right click, but if you can only double tap to initiate a right click then I see why it wouldn't ever throw. I'm planning on reworking the throw system to be a mode that's toggled on with right click and then left click can be used to throw multiple seeds.