First: How did you download? git clone ? If this does not work use the attachment I've included with this forum item
[atrol] removed attachment
I can tell you this worked for me.
Install:
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1) Extract the files to your MantisBT installation Plugin directory ( mantisbt/plugins ) so the are in "Csv_import". Rename the directory if it is not done yet
2) Then login to Mantis as Admin user
3) Click Manage, link
4) Click Manage Plugins link
5) Click "install" link after the "Csv_import" link

But all of this is also findable in the Mantis documentation, mybe you better read that first...
Anyhow..
Usage:
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Take care that this importing functionality can make a mess in your database. It is recommanded that you make a backup of your database before importing.
1) In the "Manage" menu, select "Import CSV file".
2) Select the file to import. Your file must be in a correct CSV format (see above), and must only contain importable columns. The first line of the file can be a title line. In that case do not forget to select "Skip first line". If you are using standard column names, or the name of the columns as given in your current language, it will be reconized by the importer.
3) Select the "Import file" button.
4) If the importer did not detect your columns automatically, select them. Please, always verify the columns. If you import many times from an another bug tracker, you should previously create a custom field which will contain the other bug tracker bug id. Then, check in "Primay key" column the line corresponding to the custom field.
5) Select the "Import" button.
6) Review the results to ensure they are as expected.
About CSV format :
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Be sure your file to be imported is RFC compliant!
Look at :
http://tools.ietf.org/html/rfc4180
or
http://en.wikipedia.org/wiki/Comma-separated_values