EduardoS, It's not a SP3 thing...I had this SAME exact problem and JUST FIXED IT! Do the following:
Open your applications folder
Open the Vectorworks folder
Right click the Vectorworks icon and choose "Show Package Contents"
Onced opened,
Open the folder named contents
and there your file with be (.dwg) by whatever filename you named it
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VW 2008 Student Edition
MacBook Pro