The Hollander translation (1999, reprint, most widely cited, but not always most accurate; it's simply available in the least expensive edition, so people are most familiar with it.):
Now then are come to the king's high hall,
the foreknowing twain, Fenja and Menja;
in bondage by Fróthi, Frithleif's son,
these sisters mighty as slaves are held.


The Anderson translation offers the following:
http://www.northvegr.org/lore/prose2/031.php
Now are come / To the house of the king / The prescient two,/ Fenja and Menja.
There must the mighty / Maidens toil / For King Frode,/ Fridleif's son.

Evans translation:
http://evans-experientialism.freewebspace.com/grotti.htm
Now then are come to the king's high hall
the foreknowing twain, Fenja and Menja;
in bondage by Frodi, Fridleif's son,
these sisters mighty as slaves are held.

The sisters are related to the giants, so they are definitely *not* valkyrie, as this generally puts them on opposite sides of any battle. However, they also appear to be initially benevolant, which is unlike the general character of giants in the *Edda*; they only become malevolant when their labor is abused.