To launch an application :
[[NSWorkspace sharedWorkspace] launchApplication:@"/Applications/Safari.app"];
To activate an app :
NSRunningApplication* app = [NSRunningApplication
runningApplicationWithProcessIdentifier: PID];
[app activateWithOptions: NSApplicationActivateAllWindows];
// or
NSArray* apps = [NSRunningApplication
runningApplicationsWithBundleIdentifier:@"com.bla.blah"];
[(NSRunningApplication*)[apps objectAtIndex:0]
activateWithOptions: NSApplicationActivateAllWindows];