Thanks Mactabilis! The sprite doesn't go through the platform now, except it keeps on sliding which is a bit irritating- is there a way I can replace the blocks:

If key right arrow pressed? then
change xvelocity by .1

to

If key right arrow pressed? then
change x by 3

I've already tried it and it stopped the sliding but it created another issue, the sprite goes through the platform again.
http://scratch.mit.edu/projects/18307030/