Ok. (You using Corel12? I have no idea if this stuff is the same in earlier versions.)
Short answer is, copy the file to C:\Program Files\Corel\Corel Graphics 12\Draw\GMS and (re)start CorelDRAW.
Long answer is, in the Windows 'Programs' menu under 'CorelDRAW Graphics Suite 12', there's a menu called 'Documentation'. In there is a shortcut to a PDF titled "CorelDRAW 12 Programming Guide For VBA"...more than you ever wanted to know about VBA. For now, there's a section in there toward the end on how to install a '.gms' file and access stuff inside it. There's also instructions for adding the package to the 'Tools' menu under Corel.
You need to run the "user form" for the 'fingerbox' package: that lets you change the settings for a box design.
The default settings are for my "standard" double-wall box. The drawing consists of (left to right, top to bottom) outside-front, outside-side (two required), inside-front, inside-side (two required), inside-back, top/bottom, outside-back. The green lines are the division between the box and the lid, blue lines are hinge mortises.
Try just cutting that box out of 1/4" MDF to see how it goes together. The combobox at the bottom of the dialog gives some other box types...'Demo Cube' is just what it sounds like: I use 1/8" material to make dice.
Parameters are pretty self-explanatory: "Finger Length Tolerance" makes the fingers of the joint a bit longer so you can sand/trim the burn off the ends, "Kerf Tolerance" makes the joints tighter (wider fingers, narrower slots: 0.002 is about what you want for wood).
One other note: create a blank drawing before you run the program. It will create a new drawing to put the objects into if need be, but for some reason Corel doesn't populate the normal menus...it's like it doesn't know there's a drawing active.
If you want to see what various styles look like assembled, just do an advanced search on SMC for my name using 'threads started by': pictures of all of my boxes to date have been posted there.