If you don't already have the ez-sdk install goto
http://focus.ti.com/docs/toolsw/folders/print/linuxezsdk-am389x.html
1) follow the instructions under heading
How to create an SD card
2) Then follow the instructions to boot the SD card in the EVM Quick start guide
3) You should then be able to flash you image according to the proceduring in the {EZSDK_ROOT}/psp/<xxx>-U-boot.pdf