Troubleshooting: Why won't my Locator tool enable?
Check the following:
There is not already an open instance of the Locator tool
At least one layer exists in the map
The Locator tool start up position is valid. This value is stored in the registry as Locator Tool/Top and /Left. If these values are higher than the screen resolution, the tool may be placed off of the display area.
If none of the previous fixes the problem, the Normal.mxt may have become corrupt. Close ArcMap, delete Normal.mxt, re-open ArcMap and try the Locator tool again.