once you have entered your API key in the plugin interface (by clicking on "settings" on the plugins page or in Tools -> Google maps), you can add a special marker to embed your Google maps in your posts or in your pages.
For example :
To get an url to use, like in this example, go to Google maps choose a place to show or "load" a saved map by clicking on "My Maps". You'll see at the top right corner of the map a "Link" label, click on it, it will show you two fields, get the one with "Paste link in email or IM" and copy it. Then in your post (or in your page) enter
[map:the url of the copied link] and save your article.
All should be ok ;).