When managers of Ford's Theater learned Lincoln would be attending the
performance, what did they do to prepare for his arrival?

They had cleared out the balcony for the President.

Explanation:

When the theater had found that Lincoln would be attending there was not much that they could do as they had little time.

So they allowed for the balcony to be cleared first of all for his personal use only and for providing the space for his bodyguards there.

However, this would not prove to be enough as he would be eventually assassinated in that space only and it could have been harder for the man to do it in some place else.

bless you your highness