You found a good deal on an Intermatic InTouch CA5100 Accessory Switch, you installed it and consulted the manual to set it up only to be pointed at another manual -- not included in the package -- for pairing instructions. Hopefully, you also noticed that this is a switch that doesn't actually control anything; it simply sends Z-Wave commands and reports its status (for people like me that have a regular outlet where it would have been really convenient to have a light switch).
You've searched the internet and you have no doubt discovered that Intermatic appears to have decided to pretend they never created this product. There's no reference to it on their web site and several URLs that once pointed to manuals on a different site run by Intermatic now just redirect to their homepage. Awesome.
How to PairWell, first get it all wired up. Hopefully the LED is cycling Red<->Blue. One small caveat - when pairing, the reception of the switch is significantly less than when it is functioning normally. You may need to move your SmartThings hub closer to the device to get it to work. Open the app and choose Add a Device. (I can't confirm this -- mine was in the same room -- it's just something I read several times). You can move it back where it was when you'e done. It'll start searching for devices. Hit "Up", "Down" and then press both buttons on the switch at the same time. It should show up as a Generic Z-Wave Device. Add it, give it a name and you're part way there.
Making it work
You're not quite there, yet. Though the device is recognized, SmartThings doesn't know what it does, yet. Visit this helpful forum post for a groovy script that can be added using the IDE. After you've published the script, you can visit the My Devices tab, select the switch and change its type to Intermatic CA5100.
I'm writing this post mostly for myself because I know at some point I'm going to have to do this again and after having spent about an hour trying to find some hint as to how to pair this thing, I can only imagine it'll get harder in the future. In fact, the only reason I discovered this at all was because I went to the Amazon product reviews and found someone who had mentioned the pairing procedure in passing during his review. Once that product is gone, I don't expect that information to be there any longer. As far as the device, itself -- it was inexpensive compared to others and fit my needs perfectly. Now that it's working, I have no complaints, but failing to include a manual with three simple steps to pair the thing is a pretty big omission and likely caused endless support calls.