Setting up nrf51 DK

Hi,

I am having trouble setting up the nrf51 DK on Mac OS X 10.10. I am able to download program to the board the run using Section Placement Macros of "FLASH_START=0x00018000;RAM_START=0x20002800". However, every time when the board reset/re-power, it will not run correctly. I am wondering what could be the problem? (I selected Flash_S120 in Section Placement, and I am using SDK 8.0) Thanks.