The current release version of 1.0.2 is CS5 compatible. It seems like there might be some remnant of the old version still present. Here are two things that you should try to get past this error.
First, delete the cache file, here are the steps for this:
If this does not work for you then you should try resetting the work space by going to window > workspace layout > reset 'designer or coder'
If after performing both of these you still get the error then you may need to rename your configuration folder with these steps:
Please let us know how this turns out for you.