The Collaborative International Dictionary of English v.0.48:

Appointor \Ap*point*or"\, n. (Law)
   The person who selects the appointee. See Appointee, 2.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

APPOINTOR. One authorized by the donor under the statute of uses, to execute
a power. 2 Bouv. Ins. n. 1923.