To capture Stun Break messages, you first have to make sure they are enabled.
Open the LotRO client options window (Ctrl+O), select the Combat Options tab on the left side.
Now locate the "Display combat state break notices in chat window" option and verify that it is "checked".
Once the messages are enabled, they will appear in the "Death" channel - yeah, the channel is poorly named, but that's the name that the Turbine devs gave to its enumeration constant.

To create an alert for these messages, load Alerter if it is not already loaded using "/plugins load alerter". Open the setup window with "/alerter setup".

In the setup window:
Select "New" for the Alert.
Enter a new label, such as "Stun Break".
Enter a message to be displayed, in the example I use "%1 broke %3" which will display the first "capture" which is the player name followed by the text "broke" and then the third "capture" which is the type of stun broken. "Captures" are defined in the Pattern (see below) by including part of the pattern in parentheses. More about Captures and Patterns can be found at http://www.lua.org/manual/5.1/manual.html#5.4.1
Leave the "State" options at their defaults (Normal for both In Combat and Out of Combat).
For Channel, select "Death". You can alternately select "<ALL>" but it is less efficient.
For Pattern, enter "(.+) has freed (.*) from (.*)"
You can change the pattern if you are comfortable with Lua patterns (they are NOT truly regular expressions so do not be surprised if regular expressions do not yield the results for which you are looking).
You can leave the Response tab settings at their default and simply resize/reposition the red&white border to determine where to display the alert. If you want more control over the type of alert, you can change any of the Response settings but this is not necessary for a basic stun break alert.