The Digital Sidecar's Digital IO lines each have 10kOhm pull-ups to 5V. As a result, if you're using a simple mechanical on/off switch, you can simply wire its 2 leads to "GPIO" and "GND" (the outer 2 rows on the 3x14 Digital IO header).
http://www.usfirst.org/uploadedFiles...s-Rev-0-5a.pdf
If your limit switch is more "active" (eg a Hall effect sensor instead of a mechanical switch), you'll probably need to supply power via the center pin.
If the above doesn't help, please post the P/N for your switch so that we can provide more details.
Russ