Mobile Maps is a GPS navigation software with pre-loaded maps. In order to use it, you need to download the software itself and at least one set of maps (e.g. UK & Ireland maps).

What you've probably done is downloaded and installed a set of maps but didn't download the maps software itself. Go back to the Ovi Store and download the Mobile Maps software - then you should see a "Mobile Maps 9" icon in your menu.

EDIT: the Maps software is called "Mobile Maps Application" in Ovi Store.