Optical nanoantennas can be used for coupling radiation to or from waveguides in analogy to micro- and radio-wave systems. In this letter we provide a systematic description of the design approaches for a coupler to a plasmonic slot waveguide in the telecom range around 1.55 µm with realistic excitation from a lensed optical fiber. We show that the best coupling efficiency of 26% can be achieved by utilizing a dipole antenna with side and bottom reflectors, and such coupling efficiency is 185 times larger than for the bare waveguide. The nanoantenna coupler provides a compact interface between an optical fiber and a plasmonic slot waveguide for future optical integrated circuits.