An easy way to add a stitch is to use an existing stitch. Insert the right needle into the stitch, wrap the yarn around the needle and pull through as for a normal knit stitch, elongating the loop slightly. Don't pull the stitch over the left needle yet.
Bring the top of the right needle around to the back and knit into the back loop of the stitch as shown (see the FAQ on knitting into the back loop for more detail). You now have two stitches on the right needle.