To use the parser generator first load the library code:

CL-USER> (load "libmetapeg.lisp")

then call parse with your PEG grammar as the first argument:

CL-USER> (parse "mypeg.peg" "metapeg.lisp")

the lisp code for the resulting parser will be returned.

You can save this code to a file:

CL-USER> (write-parser-to-file (parse "mypeg.peg" "metapeg.lisp") "mypeg.lisp")

To use your parser, call the parse function again:

CL-USER> (parse "input-file" "mypeg.lisp")