To summarize the other answers and comments:
Under current rules, you are not required to reveal a creature card from your hand, even if you actually have one. However, if you do not, you will not be able to cast Disaster Radius.
The relevant rule is 117.8c, which reads:
117.8c If an effect instructs a player to cast a spell “if able,” and that spell has a mandatory additional cost that includes actions involving cards with a stated quality in a hidden zone, the player isn’t required to cast that spell, even if those cards are present in that zone.
"Reveal a creature card from your hand" is an action ("reveal") involving cards with a stated quality ("creature") in a hidden zone ("your hand"). Thus, rule 117.8c says that you are not required to cast Disaster Radius, even if directed to do so "if able".
This rule was introduced in the Magic 2014 rules update, specifically to deal with cases like this:
This new rule was added to cover cases where you are instructed to cast a spell with a mandatory additional cost that includes actions involving cards with a stated quality in a hidden zone if able. That's a mouthful. For example, say you're forced to cast Disaster Radius if able. The game doesn't really know if you have a creature card in your hand or not. We don't want you to have to call a judge to verify that you're being honest about the contents of your hand. So, in the spirit of the "fail to find" rule, now you don't have to pay that additional cost and cast the spell.
Technically, before this rule was added, you would have had to reveal a creature card from your hand if you had one; if you did not, you would not have had to reveal your hand to prove this — but you might have been required to ask a judge to inspect your hand and confirm it, if your opponent did not trust your word.
Ps. The "fail to find" rule referenced in the commentary is 701.15b, which says:
701.15b If a player is searching a hidden zone for cards with a stated quality, such as a card with a certain card type or color, that player isn’t required to find some or all of those cards even if they’re present in that zone.
As Jefromi notes in the comment, this rule already covered any situation where one was told to "search" for a card, which is most of them; what it did not cover were a few oddball situations, like this interaction of Wild Evocation with Disaster Radius, that essentially told you to search for a card, but without actually using the word "search". Rule 117.8c patched that hole.