The page you've created is probably using the left navigation as defined by the Master Page - so you can't edit it. However, you can 'override' it.
Assuming that you've got the left nav and want to hide it, If you go to design view and hover the mouse over the left navigation it should say something like PlaceHolderLeftNavBar (Master). Go to the menu for that element and select Create Custom Content. This will put an ASP content control into your page - essentially, you're saying "instead of using the Master Page's content for this place holder, use the content defined in my page".
You can then edit the contents of that left navigation content control. You may wish to change the content of a few placeholders.
Alternatively, you may not have the left navigation - in this case your page has probably replaced the left navigation with empty Content controls. Find the PlaceHolderLeftNavBar ASP:Content control and delete it (thus, reverting , and make sure you don't have the #s4-leftpanel style (in the PlaceHolderAdditionalPageHead content control) defined.
Oh, and the other thing you might well need to do is click the "Advance Mode" to edit the page.