This means that the OpenLayers javascript library is not loaded on your page. At present, OpenLayers is not included in the GWT-OpenLayers module, so you have to make sure to include OpenLayers in your project. GWT-OpenLayes is compatible with OpenLayers 2.4 and up, but using OpenLayers 2.7 is adviced. There are at least two ways in which OpenLayers can be included in your GWT project. * in the head tag of your application html * in the GWT module xml
Method 1 - Include OpenLayers in the head tag of your html:
<html> <head> ... <script src="YOUR_PATH_HERE/OpenLayers.js" type="text/javascript"></script> </head> ... </html>
Method 2 - Include OpenLayers JavaScript in your GWT module xml:
<module> ... <script src="YOUR_PATH_HERE/OpenLayers.js"/> ... </module>
See also "Automatic resource inclusion" in the GWT documentation on how to link javascript files via the GWT module xml.
Read this thread http://forums.smartclient.com/showthread.php?t=4368&highlight=openlayers