You can send questions you have for a creator by heading to the FAQ tab on the project page and clicking the "Ask a question" button.
You can also message the creator privately. To message a creator, visit the project page and click on the creator's profile name. This will open their bio page where you can then click the "Contact me" button.
If you're a backer and would like your question to be public, you can post a comment on the project under the ‘Comments’ section.