This addin code is very well organized and all its power and functionality can be accessed by running one "master" function. Hence, here it is quite easy to circumvent the addin's GUI and let it work on your xml-files in a loop.
Below is one way to combine the addin with Ian's loop. Note that you may need to set the path variable, i.e. the path to where the addin is installed on your computer. Also, if expanding this code, avoid using variable names that are already used by the included code from the addin, or you may run into problems (e.g. I could not use i as a loop iterator).
// Define path variable (may already be defined)
Set Path Variable("ADDIN_HOME", "Path/to/my/Addins");
// Define all the functions of the Addin but skip the GUI
Include("$ADDIN_HOME/com.jmp.matthew.wolfe.xml.importer/xml parser III.jsl") << close window;
dir = Pick Directory("Select the directory containing your files", "$DESKTOP");
fList = Files In Directory(dir);
myInfo = [];
For(filenr = 1, filenr <= N Items(fList), filenr++,
// Use Function from Addin to parse file into a data table
dt = runParser(dir || "/" || fList[filenr]);
//Extract your information here! (e.g n rows of dt)
myInfo |/= N Rows(dt);
//Close the data table
Close(dt, no save);
);
// Did it work? Summarize in a new data table
New Table("my xml files",
add rows(N Items(fList)),
New Column("Sample", character, Values(fList)),
New Column("n rows", numeric, values(myInfo))
);