I'd like to translate a book published in 1938 by Modern Age Books,  
copyright Erika Mann, rights accessible through  the publisher.
Since Mann has died in 1969 and I can't find the above publishing
house, how should I go about getting translation rights?

That may be hard, what I can gather is that she entered a marriage of
conveince and never lived with her husband and she has no children. 
Her brother was gay and he has no children.  So there are no
decendant.  Also the publisher has since went out of business.

Two thoughts:

(1) Modern Age Books aside, I think at least one of Erika Mann's books
was published in the U.S. in the 1950s by the publisher Farrar, Straus
and Cudahy. That publishing company has evolved over the years but is
still in business today as Farrar, Straus and Giroux. It's possible
that if you contact the rights department of Farrar, Straus and
Giroux, they may have some information on the proprietor of Erika
Mann's works.

(2) Much of Erika Mann's works were surely published (perhaps even
initally) in the German language. I think the German language
publisher Rowholt Taschenbuch has or had several Erika Mann books in
print; it may be helpful to contact that German publisher's rights
department to see if they can be of any assistance.