From Webkinz Insider Wiki
The object of Polar Plunge is to get Inukshuk safely down the hill on his sled. The hills are very dangerous and you must leap over a variety of objects to make it safely down - these include snowmen (which will slow you down, but if you are slowed down too much you will lose), trees and tree stumps (which will cause a lost game every time with no exceptions), and igloos (which will cause a lost game every time unless Inukshuk is moving extremely quickly).
Use the arrow keys to control Inukshuk. The right arrow key is used to speed up, the left arrow key is to slow down, and the up arrow key is used to jump.
The Four Courses
Please see the links below in "More Information" for complete instructions and help on how to finish each of the four Polar Plunge courses.
Ice Cream Float
The Polar Plunge trophy is a randomly-awarded Webkinz World trophy - it is not guaranteed every time a player finishes a course. Instead, the trophy is triggered by finishing any of the four courses, the odds of winning being the following:
Kewl Runnings: 1 in 4 chance (once the course is finished)
Ice Cream Float: 3 in 10 chance (once the course is finished)
Polar Plummet: 3 in 10 chance (once the course is finished)
Black Diamond: 4 in 10 chance (once the course is finished)
The trophy currently sells for 50 KinzCash in the W Shop and is KinzPostable.
According to the the WI High Score Board, the highest score achieved by a WebkinzInsider member in Polar Plunge is 11,206 by member monkeecj.
See these guide threads for more help on each of the four Polar Plunge courses: