While serfty's pictorial directions are good... I actually prefer going the other way.
Turn left out of Ampang Park Station (towards the signposted Intercontinental).
Take escalator up to ground level.
Continue straight ahead past very good (for Malaysia) coffee stand.
Up escalator. Left/right across bridge to the International building. Down stairs/1 level lift. Past sports store. Right hand turn at check-in.
Avoids all the issues with the Ampang Mall. FYI The mall (first in KL) is scheduled for demolition as part of construction of a new MRT line