I have a 350 amp Black & Decker jump starter model JUS350B.Why can i not get power across the clamps with a test light

First: the Jump-Starter power switch must be in the "On" poistion, or no voltage will be delivered to the clamps to light your test light.

Second: from the Black & Decker JUS350B owners manual (http://servicenet.blackanddecker.com/documents/English/Instruction%20Manual/90553956,JUS350B.pdf?_ga=1.9223413.1518759147.1438019791)
This product has a sealed lead acid battery. For maximum battery life, we recommend the Jump-Starter be kept
fully charged at all times. Recharge before first use, immediately after each use, and once a month if not used.
Failure to do this may reduce the battery life dramatically. To check the battery charge status at any time, press
the Battery Charge Level Pushbutton.
When the battery is overly discharged and if the green LED is lit when the charger is plugged in, this indicates the
battery is at a high impedance stage.
In this event, recharge the unit and leave the unit on charge for a period of 24-48hrs before using the unit.

Also, the product reviews by consumers about this (made in China) product are abysmal:
http://www.blackanddecker.com/en-us/product-repository/products/2015/02/01/01/15/jus350b