Depends on several other packages, most of them can be obtained through Quicklisp. cl-rsasign is available at: https://github.com/mandus/cl-rsasign
In order to use the client you should create $HOME/.mcashapirc with content like this:
{
"apiurl": "https://mcashtestbed.appspot.com/merchant/v1/",
"qrurl": "https://mcashtestbed.appspot.com/shortlink/v1/qr_image/",
"merchantid": "MAPI-id",
"merchantuser": "MAPI-user",
"merchantusersecret": "secret",
"usetestbed": true,
"testbedtoken": "the-token",
"pemfile": "~/link/to/users/pemfile.pem",
"debug": true
}The pem-file is the private Mcash-pem.
I have only been able to run this on OS X thus far.