This takes place on Episode 10 - Legacy, and can be easily attained as you proceed to the mansion grounds. Continue following paths towards the main objective and you will go uphill to a base with 3 windmills and a radio tower nearby. On one side of this base, there is a steep rocky cliffside, with a small yellow crane at the top, as well as a helipad. Make your way around the base towards the top of this area using dirt paths. When you reach the top, you'll notice a small dirt path leading away from the crane/helipad towards the main objective. Follow the path and you'll find the dirt bike. Get on the dirt bike, proceed forward until you find a ramp, and jump into the mansion grounds.