As stated, there are four global instances of Inform which should be used for printing all messages in VMD. They are used just like a stream, except the manipulator sendmsg is used instead of endl to signal that the string should be printed. An example: msgWarn << level3 << "This is a level-3 warning." << sendmsg; In this case, if msgWarn is on, and it's output level is greater than or equal to 3, then this message will be printed, otherwise it will be ignored.
For debugging messages (printed to msgDebug), make sure to use the MSGDEBUG macro, so that these debugging messages can be conditionally excluded from the executable.