You've made the tree fine, you just need to make the black transparent.
Go into the 'Edit materials' menu and find the object. Then click on the correct texture. Using the 'Set Transparent' (I think) bar, change it to 'Chroma' (or something). A box should appear next to it. Click on this and it should have a colour palette. Choose the colour you want to be transparent - seemingly black - and press OK. Apply it and check the model, the black should disappear. If it doesn't, change chroma to one of the others, but fiddle around with that section and it should work.