Numbered _headings_ do show up in the navigation pane. SEQ fields do not. They are not headings although they can be used in headings (in which case they would show up as part of the heading). If you are using heading styles for your numbering though, you can skip the SEQ fields and set your headings up using the instructions here: How to create numbered headings or outline numbering in Word 2007 and Word 2010. If you use the heading styles, they can also be used to automatically generate a table of contents. They are easier to mess up than SEQ fields, though. You have to follow those instructions!

Why use Microsoft Word’s built-in heading styles?