You should set rights 755 to the folders "wp-content", "plugins", "visitors-online" and 644 to the "GeoIPCountryWhois.csv" file. In such case, the plugin’s script will have enough permissions to upload the file.
Additional Info:
http://codex.wordpress.org/Changing_File_Permissions#Shared_Hosting_with_suexec
This article related to Visitors Online plugin
Additional resources: Visitors Online - User Guide
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