Just to be clear on this...
I don't know what the original connections of the bark and leaves are. The free XFrog trees had the bark on "shader 1" and leaves on "shader 2", so I'll use that as reference here. What I did to get translucent leaves is this: Create a Lambert shader node. Link the output from the leaves node to the input of the Lambert shader. Link the output from the Lambert shader to the "shader 2" of the multishader.
In the settings window of the Lambert shader, set both colours, for difuse and for translucency tint to a reasonably bright green (a value of 0.7 or there about). Check "Tranlucent" and "Enable" and set the Translucent value to 0.5.
This should work.
If it doesn't, go to the settings of the leaves shader node and in the 'Colour' tab (the first one) set the Translucency value to 100.
That works. I tested several values, and a lot of people on the Ashundar forums have used it successfully.
Good Luck!