It would be useful to know what tools do you use (CCS/other?) I will assume CCS. The ButtonsPoll function must also be part of your project - so just #include drivers/buttons.h will not suffices. Add to the project the path to this file (in Include paths). Add the file buttons.c to your project, as a symbolic link; this way it will be part of the project, compiled and the linker will find out the code of this function. Do not forget to call also ButtonsInit() before using ButtonsPoll() (yes, you did it..).
Now, still for CCS, but should be done for other tools: to the Compiler | preprocessor add the followings defined macros:
TARGET_IS_BLIZZARD_RA1 (this will solve the ROM_xxx complaints..)
Go to TI site and download the latest version of StellarisWare - install it and in the file Your_StellarisWare_dir\boards\ek-lm4f120xl\drivers\buttons.c you will find out a description of the function ButtonsPoll(). I think it must be also described in the documents related to this board in the folder \docs, but nothing better than a look to the source code also.