The only answer that I can find seems to say that the datapower appliance only support Websphere MQ, and it doesn't understand ActiveMQ brokers.
And, the documentation for Front Side Handler mentioned Queue managers, which ActiveMQ does not have.
Is there another way in datapower to fetch/poll messages from an activeMQ?
WDP does not support all possible brokers but just a few of them (Tibco, IBM etc). If you configure AMQ to provide a REST interface you can consume messages from it using plain HTTP instead of messaging.
REST/HTTP access to ActiveMQ won't be nearly as good as traditional OpenWire/AMQP connections. It will lack support for transactions and other things, but you can at least read messages.
I do not suggest using an ActiveMQ as messaging backbone for DataPower - go for IBM WebSphere MQ instead as they are nicely integrated. If you need to occasionally pull a message or two from an ActiveMQ broker - go for the above setup.