How To Detect New Log Entry - Mikrotik Script
This script is checking if new log entry is added to particular buffer.
In this example we will use pppoe logs:

/system logging action
add name="pppoe"
/system logging
add action=pppoe topics=pppoe,info,!ppp,!debug

Log buffer will look similar to this one:

[admin@mainGW] > /log print where buffer=pppoe 
13:11:08 pppoe,info PPPoE connection established from 00:0C:42:04:4C:EE 
Now we can write a script to detect if new entry is added.

Code:

:global lastTime;
:global currentBuf [ :toarray [ /log find buffer=pppoe  ] ] ;
:global currentLineCount [ :len $currentBuf ] ;
:global currentTime [ :totime [/log get [ :pick $currentBuf ($currentLineCount -1) ] time   ] ];
:global message "";
:if ( $lastTime = "" ) do={ 
	:set lastTime $currentTime ; 
	:set message [/log get [ :pick $currentBuf ($currentLineCount-1) ] message];

} else={
	:if ( $lastTime < $currentTime ) do={ 
		:set lastTime $currentTime ; 
		:set message [/log get [ :pick $currentBuf ($currentLineCount-1) ] message];
	}
}

After new entry is detected, it is saved in "message" variable, which you can use later to parse log message, for example, to get pppoe clients mac address.