Q. Where to find short-term capital loss on 1099-DIV?

A. You don't find it on the 1099-Div.  Gains and or losses from the sale of mutual funds are not reported on the 1099-Div.  You will find that info on the 1099-B. Box 2a  (long term capital gain distributions) of the 1099-Div are  for the sales of stocks, within the mutual fund, not the gains for selling shares of the fund.

Q.  How would I be able to offset the short-term gain with the short-term loss? 

A. You're not allowed to offset your 1099-B losses directly against any short term gains included in box 1a of the 1099-DIV (gains on the sales of stocks within the fund).  Box 1a losses (if any) are treated as ordinary dividends, for tax purposes.  As such, there is only a lump sum in box 1a (no break down between short term gains and dividends).  For most cases, the difference between boxes 1a and 1b is probably short term capital gains; but that's irrelevant for your tax reporting purposes. 

Q.  Box 2a of 1099-DIV reports long-term gain.  So the short-term loss will be used to offset long-term gain? 

A.  Yes.  As others have explained, that happens on Schedule D.  TurboTax does that.  You simply enter the 1099-Div and the 1099-B.