When a stock spins off a new company, how do I enter the shares received?
You don't, use the "Corporate Securities Spin-off" transaction in that account.
It will ask for the old security's name, the new company (so don't create it ahead of time), new shares per old share, cost per old share and cost per new share.
It will then do all the appropriate calculations for each lot. In the end you will probably need to sell off the fractional share of new stock for the extra cash you received.
@craigtb if you provide the names of the companies involved in the spinoff, you'll get more detailed and specific instructions on how to enter the actions in Quicken.
