Cannot run tests

Hi everybody,
when I run "ant tests", I always get the following message:

BUILD FAILED
C:\java\keel\keel-build\import\keel-project.xml:58: Following error 
occured while executing this line
C:\java\keel\keel-build\import\keel-test.xml:65: Following error occured 
while executing this line
C:\java\keel\keel-build\import\keel-test.xml:33: Could not create task 
or type of type: junit.

Ant could not find the task or a class this task relies upon.

I tried to follow the instructions that ant gives afterwards, but 
there's only one strange thing: although ant says I might be missing 
"optional.jar", there doesn't appear to be any such file in the current 
version.

Has anybody got any suggestions?
Ciao
Maurizio