The From Address is found under your Campaign Settings. This is the value you select using the Select from address drop-down as shown below.
When your subscribers open your email, this is the email address they will see displayed at the top alongside the "From". By default, an ourmailer.com email address is created for you during setup with your company name as the alias.
You can option to purchase a customized from address to complement your corporate brand.
In the sample above you see the default from address (Metro.Candy@ourmailer.com) and the customized version (Candy@MetroCandy-News.com).