Sorry, I re-open() the Issue #1946 #1948 because I think there's a real bug, and a post is open on the forum by me few days ago https://forum.processing.org/topic/sketchpath :)
I make a new test with selectInput, that work with PDE I can open an app, but when I make an app and I try to open an other app. The target is like folder (directory).

I use Processing 2b9, on Imac 2012, OS Lion

String openPathApp = ("") ;
boolean openApp ;

void setup() {
size(300,300 ) ;

}
void draw() {
background(0) ;

if(openApp && openPathApp != ("")) {
println(openPathApp) ;
open(openPathApp) ;
openApp = false ;
openPathApp = ("") ;
}
}

void mousePressed()
{
//select Input
selectInput("Select an app to process:", "appSelected");
}

void appSelected(File selection) {
if (selection == null) {
println("Window was closed or the user hit cancel.");
} else {
openPathApp = selection.getAbsolutePath() ;
openApp = true ;
}
}