You did not set the default where clause in Data Assist Search. Edit the Data Assist Search server beahvior and set the default Where clause to 0 = 1
for reference:
http://www.webassist.com/tutorials/Checkbox-method-for-add-to-cart
Step 2: Configure the DataAssist Search
. . .
3. For the Default WHERE clause - make sure you enter 0=1
. . .