Here is the reply I received from Bookshare (with my editing to make the steps a little clearer):
Thank you for contacting Bookshare Support.
According to Don Johnston, Google changed or eliminated their dictionary look-up without notice. They intend to switch to another dictionary provider and are currently testing this new dictionary provider. Unfortunately we do not have a definite release date of when this dictionary will be available.
In the mean time, please do the following as a work around:
1. Highlight the word you wish to look up. Then copy the word to the clipboard.
2. Select the Dictionary button under the Tools menu.
3. When you receive the Google error message, select the Google logo.
4. You can then run a search in Google. Enter "def:" followed by the word you wish to look up.
5. Click the Search button and use one of the online dictionaries that appears.
We hope the above information was helpful, and please feel free to contact us if you have any questions or concerns.
Sincerely,
March Natividad
BookShare Technical Support
support@bookshare.org