If not specified, Windows will use the currently logged in user on the local box to determine permissions.
When using the first method in the sample (psExec), the -u and -p switches specify the username/password to use:
http://www.sysinternals.com/ntw2k/freeware/psexec.shtml
When using the second method (vbld_RemoteExecuteCmd), it's the 3rd and 4th parameters (from the 'system macros' help topic):
vbld_RemoteExecuteCmd(strServer, strCommand, strUserName, strPassword): Execute a command on a remote computer and return the exitcode