Use the HR Broadcasting function of your watch. Since you did not tell which model you own, you will have to look it up in the user manual for your watch. Typically you can either enable it in the settings of the concerned activity profile (probably Treadmill in this case), or you can enable it manually through the Controls menu. I highly recommend buying a chest strap, though - much more accurate, and if you buy the HRM-Pro, you will get much more data from it, not only the HR.
If you have a strap, and want to pair it with the treadmill, do it on the treadmill. You will certainly find the instructions in the user manual of your treadmill (unless you want to wait till another Technorun treadmill users peeks in, and tells you how to do it).
