Do you know the name of the file you're going after? If so, you could always just activate the window and then send the path. Something like this:
WInActivate("Open", "")
ControlSend("Open", "", "Edit1", "C:\Documents and Settings\<username>\Desktop\MyText.txt{ENTER}")
If you're unsure what the file will be, you'll need to figure out how you're going to determine file name first. You could do the ControlSend just to the directory you want, and then click on the file, something like this:
WInActivate("Open", "")
ControlSend("Open", "", "Edit1", "<path to parent dir>{ENTER}")