Sometimes 'Define Brush' option under Edit menu of Editor (Adobe Photoshop Elements) remain disabled and at times, very irritating.
Here is one of the main reasons for state being disabled - If any side of the image is more than 2500 pixels. So simple solution is to resize image to reduce each side (Width and Height) less than 2500 pixels.
There is one another similar incident through which your defined brushes get lost. This happens when you load any new brush. So best way to avoid it is to save the Brush after you define them.