Hi,
Try hitting the F3 key once you have selected the first point on your polygon. This will automatically select the whole shape, as long as it is properly trimmed. Another method is selecting your start point and cut direction and then click the end point of the shape.
If this doesn't work, try using the select chain tool and select the polygon, this will usually show you where the error is because the line of selection will stop if there is a break in your shape..
JD