How do I set up DynDNS on my MacBook?
Go to www.dyndns.com and register.  Then install the dynamic updater on your Mac and follow the installation instructions.  This will then connect to www.dyndns.com and update your DNS information whenever your Mac is connected to the internet.  You could then use SSH or other means to log onto your Mac from an iPhone or other computer/Mac by connecting to the URL (e.g. - mymacbook.dyndns.com) you select upon registering on dyndns.com.

There are another couple of steps.  You need to get your broadband router to forward traffic coming in on specific ports to your Mac.  You should be able to find a how-to on www.portforward.com for your router model. You also need to setup a static IP for your Mac which is best done on your router.  Your routers documentation should tell you how to do this.