chohole - I just had a look at the ReadMe file, and the instructions are easy enough

" to use these leaves:

• Open Bryce 5 or above
• Import the leaves (leaves.mat file) into Bryce.
• Create a new tree in the Tree Lab (use the "create tree" icon) or load a saved tree.
• After loaded click in the "E" (Edit) in your tree.
• In the area "Tree Lab" : "Foliage/Shape", choose "user leaf".
• Click in  "Edit" : "Material".  A leaf should show up.
• Click in the arrow situated above and to the right  of the leave that shows up.
• Browse the leaves that you imported before; select one of them and click OK.
• It is done! Your tree will be rendered with the selected leaves.

The important step is to choose 'user leaf'.  I just did a test with three default trees, a different leaf on each, it's worked really well.

Here's a pic of a fast render...

 

I particularly like the centre one...