When you say that your book is ready for publishing, that should mean that it has been fully edited. Fully edited does not mean you used spellcheck and so it is ready to go. Either hire a professional editor, or if you can't yet afford that, make darn sure that you do a thorough job yourself.
Spellcheck should be used, but it does not catch wrong words that are spelled correctly. It is also poor at grammar and punctuation. I find William Strunk's "The Elements of Style" a very valuable tool. Use it frequently. Also, if you are going to edit yourself, you must understand the proper use of a comma, semi-colon, etc. If you don't, research it on the Internet; and refer to your research frequently.
You should have gone through your book line by line several times. You should have looked for spelling errors, wrong words, grammar, punctuation, and story flow. Does the structure sing? Did you read the book aloud?
In order to have the same pain free experience, you MUST follow their directions carefully. The directions are not difficult, but missing a step can cause problems. Once you have followed the steps and downloaded, you will get a test view of your book. Do NOT assume the book is fine just because the first few pages look fine. Go through each page in the test Kindle or NOOK that appears on your screen. Then, and only then, should you accept it.
If later on you find that something needs to be changed, you can download a corrected copy. That also is easy, but don't rely on that and simply download the first time without making sure it is the best it can be. Every reader deserves the best you can give them.
If you struggle along the way, there are forums at both Kindle (Amazon) and NOOK Pubit (B&N) for asking questions. I have never asked a question at Pubit, as the download went quickly. I did ask a few for Kindle, because that was the first time and place I had ever formatted for publishing. If you are polite and to the point, people will be very helpful there.
You can also format to Createspace.com for a print on demand paperback. This is more difficult, but I got through it and have an absolutely beautiful book as a result. I am almost totally right brained and succeeded, so trust me, anyone can do this. It takes patience and persistence, but you will get it done.
Before you know it, you will have an e-book published and a paperback ready to print each time someone orders it. Good luck. You can do it! I promise!