short answer: you cant.

long answer: dell in their infinite wisdom chose to market this tablet with an external sd card slot knowing good and well that most consumers would assume (and rightfully so) that you could use androids built in ability to extend storage on to the sd card, a feature that is a purchase consideration decision for most, i.e. "im looking at two tablets, they both have the same features, except brand X is cheaper but doesnt have an sd card slot to expand storage, but brand Y does, but costs more, ill go with Y" sadly had i figured this out with in the "return for refund" period i would have done so. and had i known it before purchase, i never would have purchased it.

as for the file manager, they dont include one, you can use 3rd party but most of the features wont work unless you root the device, same for the sd card storage. im not recommending it. in fact if its with in the return for refund period and this was a purchase decision factor, id recommend returning it and making it clear as to why. i wish i could.

sad thing is, they could easily implament this very basic (built in to android its self) feature, with a simple update.