Hi,
as Projectserver maintains all the resources in the Enterprise Resource Pool you only can change their working time from the Resource Center view. Goto to Resource Center, select the resource you want to change and click on "Open" in the tool bar. Now the Project Professional client opens and you can edit the exceptions to the resource's calendar. Please be aware that these exceptions apply to ALL projects (which is good) and that there is no per project exception or availibility (which is bad)
Good luck!
thausla