The Climate Station is a block that manages the Greenhouse and the Cellar. When its corresponding multiblock is built correctly, it will show water on its sides. When it is invalid, it will show ice. The Climate Station must be placed on the first level of the multiblock, touching a wall. If it is not touching a wall, you may Right Click it with a block that is part of the multiblock you are trying to make to tell it what to look for.
Multiblock
The climate station in its valid and invalid state.
The climate station is crafted like this.
A note about greenhouse construction: The largest greenhouse is 31x31x31, centered on the climate station, or 15 blocks in each direction. Centering the climate station in the greenhouse will allow you to take advantage of the full size range. You can also internally subdivide a greenhouse into multiple by using walls and multiple climate stations to make it larger.