In the Build Phases of your target, click the + Add a Build Phase button in the bottom right and select Add Run Script. In the Crashlytics mac plugin, you should be given a run script to copy into this run script build phase. It will look like this:
./Crashlytics.framework/run <your api key>
Except your api key will be a 40 digit number provided by the plugin.
in your app delegate:
#import <Crashlytics/Crashlytics.h>
In didFinishLaunchingWithOptions
write the following line to start your Crashlytics session.
[Crashlytics startWithAPIKey:<your api key>];
<your api key>
is the same number in the run script.