The oven can be used if that is the only alternative. The lowest setting for most ovens is around 170 degrees F. While exposure to this high a temperature for any lengthy time may decrease some of the antioxidant qualities it can be used if it is your only option. Just turn on the oven and set to this temperature. Whole berries will not dehydrate well in the oven so the berries should be chopped first in the food processor. Spread the berry mixture on a parchment paper lined cookie sheet and place in the oven. If the berries are somewhat dry to begin with you may be able to turn off the heat once you put the berries in the oven and let them sit in the oven for two to three hours. When you remove them toss them around with a metal spatula to test for moisture. If you need to dry them more repeat this process until they are dry.
The microwave can be used for pomace (remains after juicing). Spread the pomace on a paper plate and microwave on high for 2-3 minutes. Let the berries rest and check for moisture content. Often the pomace can be spread on a cookie sheet and allowed to air dry overnight without any processing. The pomace is rich in nutrients so it is definitely a keeper!