Comet Holmes can be found in the Northeast sky near the Cassiopeia and Perseus constellations.  Late at night look high for the W-shaped constellation Cassiopeia. The W is standing on it’s side, and is about as big as your fist held at arm's length. Look about two fist-widths to the lower right of Cassiopeia. The brightest star there is Mirfak, in the constellation Perseus.  Mirfak is the top star of a smallish triangle about the size of your thumb at arm's length. The triangle's lower-left corner is Comet 17P/Holmes.

The following links provides a good example of where to look:
http://www.space.com/

http://hou.lbl.gov/~vhoette/Explorations/CometHolmes/index.html