I was having difficulty getting the UART example working from the CMSIS driver interface on a newly purchased MCBSTM32F400 eval board. After trying a number of random fixes I tried swapping the port select jumpers and it worked. The example code uses USART 3, and according to the silkscreen on the board I had to move the select jumpers from port 3/4 to port 1 to get USART 3 to work, which was a little confusing. The schematics also show port 3 selected by default, which did not seem to be the case.
I used the code described in the CMSIS-Driver interface: www.keil.com/.../group__usart__interface__gr.html