Go down on Route 216, from Snowpoint City, and look to see if you see a thing that you can use rock climb on then when you go up it will be in one of the corners, at the end.
When you go from route 217 to 216 there is a house where you can heal. From there go right past one trainer and rock climb. Then go right to the end. You can also exchange coins at the Game Arcade.
Read more: http://wiki.answers.com/Q/Where_do_you_get_ice_beam_in_Pokemon_Platinum#ixzz25PfbLKZH