I would like to use the apple watch crown to control a slider. Is this possible?
If so, how?
Apple uses it to change the colours on the UI of the watch.
EDIT: so it seems not possible at the moment (see answers below). Is important to notice that in two weeks time (Apple WWDC 2015) this could change (maybe a Watch OS for independent apps?)
Not possible. You don't have the crown in control – it's used only automatically for scrolling etc..
Apple guidelines:
"Scrolling is the only supported Digital Crown interaction for apps, and the system automatically manages those interactions for you. Apps do not have direct access to the Digital Crown."
https://developer.apple.com/watch/human-interface-guidelines/
Apple might enable digital crown when the Apple Watch native apps are out (we will see in 2 weeks at WWDC).