An event with so many people at it will definitely be on their radar. It seems fairly unlikely that they'll be successful, because of all the security. However, they might try. Having said that, there are so many big events that one would expect ISIS to try to attack, and most of the time it doesn't happen.
From Zika to poisonous, garbage-filled waters, Rio has had its hands full trying to get the Olympic games off the ground. Another issue they should address is the threat of ISIS: with large crowds and everyone watching, it is entirely possible that the terrorist organization will use the games as a stage for further horrific activity.
The recent activity of ISIS is evidence of the organization's geographical and ideological reach. From Pakistan to Paris to San Berdnardino to, most recently, Orlando, it seems that no country or town is immune from the group's violence. The ISIS connection in tenuous in many circumstances, but that does not lessen its strength. Individuals claiming to have pledged allegiance to the group, or who wish to act on the groups behalf, can do so without the direct knowledge or support of the main body. Medusa's snakes are everywhere. Given this, along with the reach of the "head" of the organization itself, an attempted ISIS-related attack in Rio seems almost assured. As for the source, scope, and success of any such attack . . . the world will find out soon enough.
I wouldn't put anything past ISIS, but I would hope that the Olympics would have plenty of security to prevent any major attacks from happening. Given that incidents have happened in years passed at the Olympic games, one would think that they already have fairly tight security anyways to prevent terrorism.