There's no help document that I know, just a quick introduction the first time you open Stackedit in a new browser. However, for your question about Dropbox, I happen to know from using various apps with Dropbox, that if you want to publish to the root of Dropbox, just type "/". If you want to publish to a specific folder or subfolder on Dropbox, you would type the path to that folder, starting with "/" and with another "/" for every time you go into a subfolder. So if you have a folder called "Cats" inside a folder called "Animals" and the "Animals" folder is one of the root folders on Dropbox, and you wanted to publish in the "Cats" folder, you would write that as "/Animals/Cats" (without the quotation marks).
Stackedit documents are created in your browser's local cache. If you start a new session and that involves your browser clearing its cache, then your Stackedit document will be gone, unless you synced it to a cloud service such as Dropbox first.
Further, if you open Stackedit in a different browser application, it will not load your document automatically, because each browser application (Chrome, Safari, Firefox, IE, etc) has its own cache.
If you synced a document to a cloud service, you can open that document again by clicking on the "#" in the top-left, then choosing Synchronize, and then choosing "Open from [your cloud service of choice]."