Learn German: Das vorzubereitende Referat kostet uns viel Zeit. - The presentation that is to be prepared costs us a lot of time.

This is the Gerundivum (sometimes called the zu-participle). For a separable verb like vorbereiten, you insert zu between the prefix and the stem (vorzubereiten), add a -d to make it a participle (vorzubereitend), and finally attach the standard adjective ending. Because it modifies das Referat (a neuter, nominative noun following a definite article), it takes the weak ending -e.

What exact meaning does the Gerundivum add here, and how else could I say this?

It acts as a passive adjective of obligation. It means the presentation must or needs to be prepared. You can think of it as a formal, compact way of writing a relative clause with a passive modal verb. You could express the exact same meaning like this: Das Referat, das vorbereitet werden muss, kostet uns viel Zeit.

Is uns in kostet uns in the dative or accusative case?

It is accusative! The verb kosten is one of the rare German verbs that takes a "double accusative." It requires an accusative object for the person experiencing the cost (uns) and another accusative object for the price paid or time lost (viel Zeit). Because uns looks identical in both dative and accusative, learners often mistakenly assume it's a dative indirect object.

Can I use Referat to talk about a presentation at work?