How do you get the skateboard skill in Sonic Generations?

Answer 1

The skateboard is not a skill. It's only available in City Escape. In the Classic stage, the skateboard is in an item box and remains with you until you get hit or lose momentum. In the Modern version, the skateboard is with you in the beginning of the stage, alike to the original version, until Sonic hits a tram and jumps off the board.

It is not available anywhere else.

Answer 2

The skill shop at misson select at the far left is how you activate them.

just equip um