How you message the creator of a project you have backed will depend on which device you are using to access Kickstarter. You can find the steps for each below.
From your desktop or mobile web:
Go to your Backed projects page
Scroll to the project whose creator you wish to message
Click the round “+” button to the right-hand side of the project name
Select the 'Messages' tab and send a message to the creator
To message a creator from the iOS app:
Visit their project page
Tap on the creator's profile name to open up their bio
On the bio screen select "Contact"
Next, tap "Ask a question" to send the creator a message
To message a creator from the Android app:
Visit their project page
Tap on the creator's profile name to open up their bio
Next, select the mail icon in the top right corner
On the next screen you will be able to send a message to the creator