You can only receive the in-game rewards AFTER you have linked your Twitch account to your Guild Wars 2 account, and only WHEN campaigns are active. You can find more information about Twitch Drops and account linking here.
When you meet the campaign requirements, but have not received your Twitch Drops, you can claim them from your ArenaNet Account page. You will need to log in and make sure you are on the Applications tab of your account page.
1. Under the Account Linking panel, click 'Check for undelivered Drops'
2. If you have undelivered drops, a pop-up box will appear.
If you continue to experience problems with Twitch Drops, contact the Support Team.