If the polyline is truly a polyline, and all one piece, simply type "list", hit enter, select the polyline and hit enter again. The text window should pop up, (if not hit f2). You will get a bunch of info about that polyline, including length. It will look kind of like this:
LWPOLYLINE Layer: "0"
Space: Model space
Handle = 3346
Open
Constant width 0'-0"
area 435864.360 square in. (3026.83583 square ft.)
length 233'-8 7/32"
at point X=147'-4 93/128" Y=21'-8 33/64" Z= 0'-0"
at point X=147'-4 93/128" Y=75'-6 107/256" Z= 0'-0"
at point X=213'-1 69/256" Y=75'-6 107/256" Z= 0'-0"
at point X=213'-1 69/256" Y=133'-1 17/32" Z= 0'-0"
at point X=269'-7 119/128" Y=133'-1 17/32" Z= 0'-0"
This can't "not work" unless it is not really a continuous polyline.