Open Crimson, go to the Menu Bar, and click Document, then look in the menu for Small C (it should be there) and click it.. now your document will be Small C syntax highlighted..
If you don't have Small C in the menu, then click on Customize at the very bottom of the menu, and add Small C.. All you have to do is copy the .spc and .key files to your 'C:\Program Files\Crimson Editor\spec' folder and point to them from the customize dialog-box..
For updated Spec files, EKS has posted a link
here