This can be unlocked at any time, online, offline, in any race mode.
The Crash TV Ski Jump is located not far from the junkyard on the dirt tracks, found by driving up the dirt round located at the east side of Grange Hill, behind the dirt ramp are two paths with Mega Jump cones, the path to the left leads to the Ski Jump.
Make sure you have a full boost tank before jumping and try to keep the car straight off the ramp to avoid crashing when you land.
Here's a video demonstration.