Since the entire distribution is taxable you would only enter that amount on 16b. Line 16a will be left blank. You will enter the amount of taxes that were withheld on line 64. The 10% penalty for withdrawals prior to age 59 1/2 does not apply to distributions from 457 plans.
See instructions on page 22 - http://www.irs.gov/pub/irs-pdf/i1040.pdf
Enrolled Agent
EA, QPA, CHFC, CEBS, CLU - 29 years experience providing financial advice