Put the number in the search box and hit enter? Don't use the #, afaik.
If that dosen't work, make sure the rest of the fields are blank and you aren't hiding played/rated arcs?
"I think you're confused. This is /b, not /b/."
I had this question last night. And obviously any question that Zube may have cannot, by definition, be "stupid." 
Open the MA ststem interface.
In the upper left corner it says "Browse and Play."
Right below this it says "Search Options." Click this.
A space appears in which you can type keywords or arc numbers. Put the number here and click "search."
"OK, first of all... Shut Up." - My 13-Year-Old Daughter
29973 "The Running of the Bulls" [SFMA] - WINNER of the Mighty Big Story Arc Contest !
- The Stellar Wind Orbital Space Platform
Thank you. That did not look like a search box to me, but the Zube came through.
I've tried to find missions by number--but can't figure out how to do that.
What am I doing wrong?
Thank you.