With Dex, you can add a friend's birthday manually through the contact detail panel. However, Dex automatically adds birthdays during the Facebook Import.
Once your birthdays are imported, you can easily access them from the Birthdays page in Dex:
You can also add birthdays for individual contacts in the contact detail panel.
By default, most users will only have reminders via email for Most Birthdays, not all birthdays. In particular, you can configure your birthday reminder notification settings here:
None: (if you turn this off, you won't get any reminders)
Most birthdays: you'll only receive email reminders for contacts that are starred or have a last interaction.
All contacts: you'll receive email reminders for all contacts that have a birthday
Looking to add other types of reminders?