Failing to enter Navigation is a common fault with late model Binatone Satnavs running Windows CE 6.0.  I'm sorry to say there's no solution.  I've got two (G430 and a B430) with this problem and I can't fix them.  
Suggest you report it to Binatone and see if they'll give you a replacement.
                
 Stephen R
                
 November 2010