The input for this is very obscure, unfortunately.  So, not surprised you couldn't find it.

1) Go to Income > Passthrough K-1's > Partnership Info (1065 K-1);

2) Click on the State & Local tab; and

3) Under the State Adjustments and Credits section, select the following:
     (a) SCREEN: 55 = 55 = Partnership K-1
     (b) INPUT CODE: New Jersey > NJ308 = NJ308 = NJ nonresident share of NJ tax
     (c) AMOUNT: Whatever the amount showing on the K-1 is.

The amount from 3(c) will flow through to NJ-1040NR, Line 52 and the details will also be populated on Schedule NJ-BUS-1, Part III.