It really depends on the age of the saw. There's a switch either on the top or end of the arm (older model) or just behind the handle on the motor itself, there is a flip switch that you pull outward. It should have a small (yellow) plastic insert in it that acts as a safety key and prevents the saw from being started by children, etc. If you see a slot in the switch with no inserted clip, the safety device has been lost and you will have to replace it in order to run the saw. If you need to replace the switch, search google for "sears craftsman radial arm saw safety switch" or see this link.
You may be able to get a manual for your saw at this link,
depending on its age. Otherwise, you might call sears service center or check the new saws and see if a comparable part will work with your model. I have also had good luck finding old manuals at this site.
I hope this information allows you to resolve this issue. If you need further assistance, please post back with a comment to this thread.
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