Take Metro B line all the way to Circus Maximo, get off here and start walking to Mouth of the Truth, visit Vesta Temple on the left, follow via Petroselli and you'll meet Teatro Marcello also known as Small Colosseum keep going upward and you'll see the Capitoline Hill - the Major's house - enter this place and go all the way to the back where a huge area will get open to your eyes: you're on the Imperial Forae. Head shortly to the Colosseum where you'll find the entrance and if it's not too late get into the Colosseum.