Whether or not to get an ISBN is a tricky question. It can depend on country (some countries charge for ISBNs, others don't), on what you want to do with your book, on how you want to track your book's sales, where and how you want your book reviewed, etc. etc. etc. We have also heard contradictory accounts of whether or not you need a separate ISBN for each ebook format in which you publish your book.
Briefly, here is the Leanpub position on whether or not you should get an ISBN:
You definitely do not need an ISBN until your book is finished. So while you're writing and publishing your book-in-progress using Leanpub, we recommend you don't worry about ISBNs - just focus on your writing!
Start worrying about getting an ISBN only when you discover you absolutely need one. For example, if you are going to publish your book using other 'channels' like Kindle Direct Publishing or Lulu or CreateSpace, under certain circumstances you will be required to provide an ISBN. At that point, you'll know you need one, and you'll know why, and you can ask whoever's requiring the ISBN what they need.
To add an ISBN to your book, please follow these instructions: http://help.leanpub.com/author-help/how-can-i-add-an-isbn-to-my-book.