Ian, your first mistake was buying Proteus, they are to quality what excrement is to healthy eating...
Secondly, how can this be confusing, they have two terminals, one at the top and one at the bottom. The Bottom Terminal will sit on the Busbar of the unit it goes into, or if no Busbar the supply TO the MCB will go into the bottom terminal. The Feed to the actual circuit the device protects will go into the terminal at the top of the MCB.
To be honest though fella, if you need help wiring an MCB up, then you should not be playing with anything electrical.
You say it is a new garage . . . this would lead me to believe you are in fact fitting a full consumer unit with several MCBs within it, hence your confusion, since this is a slightly bigger job than wiring an MCB.
I don't always see eye-to-eye with FWL, but this time he's in complete agreement with me - if you need instructions to wire up an MCB, or a CU, then you shouldn't be doing it.
In fact, I'm not aware that MCBs or CUs come with instructions, do they?
Finally, there is the little matter of Part P for you to consider.....
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