The Edinburgh club looked set to hold on for the win at Tannadice after John Rankin's early opener was cancelled out by James McPake and a Leigh Griffiths strike put the visitors 2-1 ahead.
But United claimed a share of the points when Johnny Russell converted from 12 yards with just four minutes to go after Ryan McGivern fouled Gary Mackay-Steven.
Television replays showed the challenge took place outside the box but when asked for his thoughts on the penalty call, McNamara said: "I've not seen it again.
"I thought there was a foul on Johnny before it that we didn't get.
"Some people are saying it was outside the box but I thought we deserved at least a point for the second-half performance."