Map Reporter is accessed via the More icon on the main screen of Ovi Maps. Depending on where you are located, a different number of services will show up (e.g. in the UK you'll see Qype, Expedia, and Timeout as shown below).
When you open Maps Reporter, your current location will be automatically entered (though this can be edited). You then choose what type of report you would like to make (missing street or path, incorrect address, incorrect one way restriction, blocked street or path, not suitable street for walking and poor street lighting).
This generates an automatic report, which can then be submitted to Nokia. You can optionally choose to add additional details (it's probably a good idea to describe the error you are reporting here) and supply your email address.
It is worth noting that Ovi Maps does already include some automatic 'map improvement/reporting' functionality. In the settings of Ovi Maps you have the option to turn Maps improvement on or off (shown below). Presumably this sends back data to the Ovi Maps/Navteq/Nokia servers, which helps improve mapping data.
However there's clearly an advantage to having manual reports as an option too, as humans are generally going to be far better at picking up and reporting certain types of errors.
It's interesting to see Nokia adopt crowd sourcing as a source of data for improving its mapping data. With tens of millions of active Ovi Maps users, there's a huge potential for getting error reports. Moreover, the mobile device is probably the ideal reporting tool as it allows users to submit reports immediately, in contrast to Navteq's current Map Reporter, which requires access to a PC.
Rafe Blandford, AAS
Thanks to @andrew_heatley for the notification.