I've build a multifarm:
However, when I right click the farm blocks no GUI appears... So how do I access it?
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Remove the dirt, the farm places humus itself.
The farm also has to be exactly four blocks high. The horizontal size depends on the size of your multifarm. All possible options can be found here.
Keep in mind that you have to constantly supply power and must regularly insert water, dirt and fertilizer. For maximum effiency it's best to build the stone bricks higher up against the farm center (not above the center though), so your machines for input/output of resources don't hinder treegrowth.