Hey Sneak!

You should automatically unlock hardmode once you've completed all the normal stages. It could be that you haven't gone on the hard map yet which can be found on the bottom left of the campaign map. If you still can't figure it out, could you send a screenshot of the finished normal stages and then another one of the locked hard stages?

ok I got it after relog. 

Thanks

I'm having same problem. I have beat the first campaign got 3 stars in all stages but can't unlock hard mode. I logged from Facebook and logged out and in but no luck.

Same, I have done everything, but hard mode is still locked.