Is mourn a word in the Scrabble Dictionary

Yes, mourn is playable

...and is worth 10 points.

Found in the following dictionaries

Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)

"Mourn" Definition

verb-transitive

To feel or express deep regret for: mourned the wasted years.
To grieve over (someone who has died).
To utter sorrowfully.

verb-intransitive

To feel or express grief or sorrow. See Synonyms at grieve.
To make a low, indistinct, mournful sound. Used especially of a dove.
To show grief for a death by conventional signs, as by wearing black clothes.

verb

To feel or express grief or sorrow.
To show the customary signs of grief for a death.
To wear .
To murmur .
To feel or express grief or sorrow for.