May I ask exactly why you chose dynamic. A basic disc is fine for normal use. You would normally only see dynamic discs in servers. By the way, if you ever want to change it back to basic in the future the process is very simple, but totally destructive. All data will be lost. I would decide now before you put anything importnat on it. For more info on the differences click here
My advice, change it back to basic (right click and choose convert to basic disc), then right-click the unallocated space and choose format, give it a drive letter when asked and let it run through the process.