She's found in Alternate Stage 4. Simply keep going through the stage (do not go down the waterfall on the raft yet). Go all the way across this part of the stage until you notice a blue frog. Don't kill it. Instead jump over/avoid it and let it stay close. It will follow to the end of this particular part of the stage to a row of frog statues. It will jump on the only pillar that doesn't have a frong statue on it and lower a plateform in the middle of this section of the stage. Go down the stairs and follow the path until you get to the end where there is a gate. Iris is awaiting there.