He was told if the party legally bought the music from a production library and it held an Apra Amcos licence, it would be fine.
He said he was "embarrassed" to be asking the different groups about the safety of using the track.
"[They] couldn't understand why I was asking about the appropriateness of using production music if they come from a recognised production library.
"I think they though I should have known this, with my experience in the industry."
De Joux said continued media comment on the song being similar to Lose Yourself was "unhelpful at a time when the party was trying to publicise its policies".
The party decided to change the music to keep the ad from distracting people from the campaign.
Yesterday music producer Jeff Bass gave evidence. He was the person who composed the original guitar riff for Lose Yourself.
He played the riff to the court on an acoustic guitar, before stating Eminem Esque was "a blatant rip-off" of the song, and was "like Lose Yourself Lite".
During the defence opening, lawyer Greg Arthur said the party paid $4802 for the licence for the track.
He said the song "no more than blandly referenced Lose Yourself."
"It's dangerous to be lured into listening to two pieces of music and thinking 'oh, that sounds a bit the same ... The copyright question is what is original in Lose Yourself and is there reproduction of a substantial part of that."
Arthur said he would call a witness who would say certain parts of Lose Yourself were unoriginal, so the copyright could not be infringed.
The trial will continue tomorrow.