When looking at the synonyms tab in the schema browser, there are no synonyms listed.
Turn filtering off to make sure they are not filtered out.
NOTE: Make sure the "Through a DB Link" is NOT selected in the FOR OBJECT TYPES section of the Filter window; if turning off filter does not work.
If you are in treeview, switch to tabbed view. You can do this by using the Browser Style button located on the right side of the schema browser window. Synonyms do not display in treeview.
Upon further evaluation, it has been determined that the issue will not be remedied.
Update Dec 2010:
ST80340 has been created for the issue where Synonyms are not shown in Treeview mode, but do show in Tabbed Mode. This is also related to which synonyms are public and private and Toad only showing private synonyms in Treeview mode. This should be addressed in Toad 11 beta