This is an obsolete controller which also has obsolete tools.
Not much is known about how to modernise development with this.
Version 10.0.0 would appear to be the last SDK for just the nRF51 chipset, and 12.3.0 appears to be the last that includes the nRF51.
Here is an overview.
Go to the download page and fetch 12.3.0.
After unzipping the following files are found:
nRF5SDK1230.zip s130nrf51201.zip s132nrf52310.zip
`s130nrf51201.zip' is the BT software stack for the nRF51 (known as the Softdevice) and `s130nrf51201.zip' is the SDK for nRF5x.
The Softdevice stack for the nRF52 named `s132nrf52310.zip' is not needed here.
The GNU makefile `components/toolchain/gcc/Makefile.posix' in the SDK sets up GNU_INSTALL_ROOT := /usr/local/gcc-arm-none-eabi-4_9-2015q3 and this toolchain would appear to be an absolute requirement.
This toolchain is available here, but it's 32-bit for i386 only.
Not sure what to do with this yet.
md5sum s130_nrf51_2.0.1_softdevice.hex f63d43ae56cada5d5aaa14271fc76681 s130_nrf51_2.0.1_softdevice.hex
This is used to program the Softdevice and firmware, somehow.
https://www.nordicsemi.com/Products/Development-tools/nrf-command-line-tools/download