Place down the holographic projector, and right click (without any tools) to bring up the interface.
1 slot -- Place an item in here, and that is what the Hologram uses to create an image. Example: Place smoothstone, and the hologram is made of smoothstone. Place Cobble, etc you get the idea.
3 lines for settings, with numbers 1 though 9. This is Height and Width and depth. This tells you how big the projector's image will be. Example:
You choose 1x3x3, it creates a 1x3x3 wall of whatever material you placed in the item slot. (Not sure if this is do-able, if not, we'll need to figure out another way to size the thing).
Finally, theres a checkbox (or yes/no, or something) that determines whether the hologram is 'solid' or not. Solid holograms act just like normal MC blocks -- non solid holograms LOOK just like MC blocks, but you can walk right through them. (Perhaps toggle this with a redstone current?)
Requires 1eu/s per block. 2 eu/s per SOLID block (double energy cost to make image solid, aka can't walk through it).