Check out line 564 of the code for wiring info. Here’s a pic as well:
http://docs.spark.io/#/firmware/communication-spi
Line 551 is the start of the user application, everything above that is the library.
https://gist.github.com/technobly/ba306404a1a90554eb59#file-sparkliquidcrystalspi-cpp-L551
Make sure you copy the raw view of the code:
https://gist.github.com/technobly/ba306404a1a90554eb59/raw/
You shouldn’t have to define any pins as long as you hook up your 74HC595 to A2, A3, A5 as pictured. A4 is not used.
You also have to make sure you hook up the pins from the '595 to the LCD like this:
Good luck!