If you have a flexible faucet hookup hose, you won't be able to use the inlet saddle.  One alternative is to remove the hose from the outlet stop (this is the valve that is attached to the water pipe coming out of the wall) and insert an adapter which has an outlet for reattaching the hose, but also has an additional 1/4" outlet for the RO system.  You can buy one at a hardware or plumbing store, but you will need to know the size of the hose going to the faucet (usually 3/8").  Don't forget to put teflon tape on the threads before connecting the various parts.

A second alternative is to replace the outlet stop with a dual outlet stop.  One end of this piece screws onto the pipe coming from the wall, but it also has an outlet for the flexible hose and an additional 1/4" outlet for the R.O. system.  You can buy one at a hardware or plumbing store, but you will need to know the size of the pipe coming from the wall (usually 1/2") and the size of the hose going to the faucet (usually 3/8").  Don't forget to turn off the water to the entire house before beginning this project and put pipe joint compound on the pipe threads before screwing on the dual outlet stop.