1) Make sure the contact isn't unsubscribed, and that he/she has a valid email address.
Go to the Contacts page (fourth icon, first link) and search for the contact by email address in the top right. When you find the record, double-click to open the Contact Details window.
You'll see a series of links at the top of the window; click on Preferences. You can view/change subscription preferences at the email category level on the left side (email category changes don't require saving), and global status on the right side. Also found on the right side is the email address status. If it's marked as invalid and you know that it's a good email address, you can click the Reset Status button.
NOTE: If you see 'No Status' by an email category, it simply means that no action has been taken either way (i.e. the contact has chosen neither subscribe nor unsubscribe), and the contact is opted into that category by default.
IMPORTANT: You are NOT allowed to change subscription preferences without permission from the contact.