file_dialog is good if you want a popUp Window (like Windows "open file")
For an ingame option use:
Create a folder at runtime (or create one by hand with rightclick-> new in the project folder):
CreateDirectory(_chr(str_cat(work_dir,"My Box Folder")),NULL);
and use
txt_for_dirto get a list of all files (with specific extention) of the folder.
Now just create clickable buttons with PANEL´s button function and digits, and you´re done ^^