Learn Dutch: De media zijn op de hoogte van de misstand. - The media are informed about the malpractice.

Literally, it means 'at the height'. However, the fixed expression op de hoogte zijn van means 'to be informed about', 'to be aware of', or 'to be up to date on' something.

Why does the sentence use zijn instead of is after media?

In Dutch, media is strictly a plural noun (the plural of medium). While native English speakers frequently say 'the media is', Dutch grammar requires the plural verb zijn.

How is the word misstand formed?

It is a compound of the prefix mis- (wrong or bad) and stand (state or situation). It literally describes a 'bad state of affairs' and is used for malpractices, abuses, or systemic wrongdoings.

Since this lesson is about evidential clauses, how would I add a phrase like naar wij aannemen (as we assume) to this sentence?

If you put it at the beginning, it acts as the first grammatical element and triggers standard V2 (verb-second) inversion. The verb zijn must come immediately after the clause: Naar wij aannemen zijn de media op de hoogte van de misstand.