Learn Croatian: Morate ići u ured. - You must go to the office.

Because it follows the modal verb morati (must). Just like in English where we say "must go", in Croatian you conjugate the first verb (morate for "you must") and leave the second verb in the infinitive (ići meaning "to go").

Where is the word for "you" in this sentence?

It is built into the verb! The ending -te on morate tells us the subject is "you" (plural or formal). In Croatian, you can usually drop the pronoun vi (you) because the verb ending already makes it clear.

Doesn't the word u usually mean "in"? Why does it mean "to" here?

The preposition u means "in" when you are resting somewhere, but it means "to" or "into" when you are moving towards a destination. Since ići (to go) is a verb of motion, u ured translates as "to the office".

Is the word ured in the accusative case here because of the movement?

Yes! When u shows direction or movement to a place, the noun that follows takes the accusative case. For a masculine noun that is an object (not a person or animal), the accusative form is exactly the same as the basic dictionary form. That is why ured does not change its ending.