Quick answer:
Select Messages on the left of your Parent Account to read messages from the school. Use Leave a Message to reply and Attach Files to share documents.
View messages by logging into your OpenApply Parent Account and clicking on Messages on the left sidebar.
You can find sign-in instructions in Accessing your parent account.
Sending Messages
You can send a message to the admissions office via Leave a Message and share files via the Attach Files button. When you are done, click Send Message.