Scroll down to the bottom of the page and click on the link that says, "Dynamic Spine Calculator Rev 12-25-10 (v2)
updated 2-14-2011."
Click yes to save or whatever the window says. After the download completes, go to wherever you saved it (probably My Documents > Downloads. Open the STU file there and follow the install instructions. Actually, now that i think about it, i don't think there is any installation process - you just open it from wherever you saved it. You should be good to go after that.
If you don't have Microsoft office, download (free) Open Office here:
http://www.openoffice.org/ It's basically the same install process. Open Office will handle all Microsoft office files.
As far as actually using STU's, there are good instructions.
*Edit* I see I was late with the open office suggestion. I'm typing on a phone.