> "Pick up the hammer."

Well, "pick up" is basically 'taka upp'.

"Skrímnir tók upp hanzka sinn."
(Skrímnir picked up his glove.)

A completely literal translation would thus be:

"Tak upp hamarinn!"

Zoëga quotes Snorra-Edda as saying:

"[Þórr] tók upp hamarinn Mjölni ok vígði hafrstökurnar."

http://www.northvegr.org/zoega/498.php

It seems some other editors omit the 'upp'. I don't have the
texts of the mss but it seems likely that there is both an
upp-version and an upp-less version :)

If you don't like the attached article (it was probably not used
much until after the composition of most of the Eddaic lays) you
could use a demonstrative pronoun instead:

"Tak upp hamar þann!" (Pick up that hammer!)

"Tak hamar þann!" (Take that hammer!)

A (weakened form of) the 2nd person pronoun is often attached to
the imperative in Old Icelandic (and almost always in the modern
language).

Tak þú upp hamarinn!
Taktú upp hamarinn!
Taktu upp hamarinn!

Kveðja,
Haukur