To edit your Waiting Room, click the "Waiting Room" link on the left sidebar on your dashboard. If you hover over what's in the waiting room it gives you the option to delete it. Click the + to add text, images or video and your patients will see it when they are waiting to visit with you. Make sure to hit the "Save" button at top once finished.
To change the greeting when someone checks in: Go to "Account Settings", under "Personal Info" and change "Title" to "Other", and then your last name to whatever you'd like it to say. For example, if you'd like it to say "...let us know you are here" you would change your last name to "us".