Your alternative 'from addresses' can be found under Profile > Change Email - section Alternative Email Addresses -
If you don't have any alternative sender email addresses listed here you can easily add some.
How to add a new 'Sender Email Address'?
You can access the alternative sender's email menu via your Profile.
Once you've accessed the menu, all you need to do it is go to the Change Email tab and then navigate to Alternative Email Addresses. Here you can add and/or remove the alternative sender email addresses.
After you've entered the email address you'd like to use just click Add.
You will be sent a verification email to confirm that you control it.
NB Make sure to check your Spam folder for the verification email.
After you've confirmed your alternative email address it will be added to your list and you will be able to select it when sending campaigns.