First you need to have your email username and password. If you do not have these please contact support.
Next, in your web browser go to http://www.yourdomain.com/login (the /login part is the key here).
You will then be prompted to enter your username and password to login.
(uncheck the box for secure connection unless you know your company has a secure certificate)
Once logged in you will want to click “email” on the left side menu. You will then see the section for a vacation message along with a start and stop date/time. Make sure to check the box “enable” and setup the times and your message and then click the “save” button and you will be all set.