Go to the folder with your wallet.dat and blockchain files in it.
Edit or create bitcoin.conf
Put these 3 lines in it:

rpcuser=yourusername
rpcpassword=yourpassword
server=1

Then change the script so the user and password at the top match your bitcoin.conf:

$btcu = array("user" => "yourusername",             // RPC Username
            "pass" =>   "yourpassword",               // RPC Password
            "host" =>   "localhost",      // RPC Hostname/IP
            "port" =>   8332);            // RPC Port

Then restart bitcoin-qt.

Pick something other than 'yourpassword' of course - so long as it's the same in the bitcoin.conf as in the script, it's ok.