There are two Training Areas in Dragon Block C:
King Kai's Planet
Hyperbolic Time Chamber
Both have a -9 modifier to strength and dexterity, which means you'll be slower and weaker. They help with training, though King Kai's Planet cant have a safe zone if you want to train with the shadow dummy.
Hyperbolic Time Chamber is easy to reach. Simply use the Nimbus or Flight to make it to Kami's Lookout, which is always at coordinates 80 220 60, if you wanna tp there. Just fly above korin's tower, and you'll easily notice it. Once you're on the lookout, go through the large dome in the center with Vegeta and Trunks. Once you're in the dome, go through the door in front of you, and you'll be in the time chamber! You can only be in it for one day, but it'll be good training.
King Kai's Planet will be much harder to reach. The first step is to die. I'm not kidding. Put your inventory into a chest, and take a long bath in lava. If you have your config files set right, you should have the death system on. If it's on, you'll be in otherworld. It may seem easy to get to King Kai's, but it's a long trip ahead. To get there, ignore Yenma. Just keep walking until you reach the giant snakehead. Once you're at it, use more power until you're able to climb it. Once on top of it, you'll next need to run Snake Way, which is around 3000 blocks. Once you reach a dead-end, you better have flight, or better drop down into hell to get blocks. You'll need to fly up to King Kai's planet, or build your way up there. Once up there, congrats! You made it! If you're using the /tp command, you'll really want to record the coordinates for King Kai's Planet.