Add the following to config.txt:
dtparam=spi=on
config.txt has every setting with line feed in between, so every setting must be on it´s own line.
If you are on Windows, do not use notepad as it uses the wrong line feed for Linux (CRLF instead of LF), use something like notepad++ or similar.
If you are on OSX or Linux you are good to go with pretty much any text editor.