Generally speaking when there is a problem with an installation I first suggest deleting the cache file.
common_installation_issues.pdf
If this does not work for you then you may need to rename your configuration folder and have DW recreate it for you. Here are the instructions for this:
showpost.php?p=12293&postcount=2
If you continue to have the problem please let us know and also include details about your OS and the extension manager you are using.