Grammatically speaking, it may be argued that both are correct but the second sentence ("The presentation was revealed") I would suggest is the more common phrase in everyday speech.
It is a past passive sentence, which means the action and not the doer has our main attention. We are focussed on the result, the revealing of the presentation, and not who performed it but before we can say why this is the better phrase we need to know when we use the present or the present continuous passive tense.
Simplifying; the present simple passive we normally use for actions that are always true or can be repeated. For example:
- Coffee beans are grown in Columbia.
- American whiskey is exported to over 100 countries world wide.
- Milk must be kept in the fridge.
The present continuous passive for actions that are in progress or are actually happening at this moment.
- A new motorway is being built. It should be ready by the end of spring.
- The children are being looked after (by my mother).
- My car is getting fixed.
In the case of "The presentation is revealed" versus "The presentation was revealed" I would find the present tense odd because although presentations can be repeated, the act of revealing a presentation is by its very nature brief and therefore, unlikely to be prolonged. Furthermore, it is not a permanent situation (the butter must be kept in the fridge) nor is it repeated (American whiskey is exported...).