installing through wp-dashboard makes file/folder ownership www-data and denies all ftp accounts acc

Please please help me, I'm at the end of my tether with this. I've tried the Plesk forums, my server host and ENDLESS Googling and it all seems so complicated and beyond me! I can't even find search terms specific enough to solve the problem.

I tried to install the Edublogs bbpress theme and discovered I couldn't upload to bbpress folder as it gave me a 550 permission denied error. I look further into this and discover that whenever I install any plugins through the wp-dashboard (instead of manually uploading via ftp), Wordpress assigns the files and folders for that plugin/theme to the user 'www-data'.

(The permissions for anything uploaded via wordpress to the user 'www-data' are set at '755' by default if this is relevant?)

I can't access this user and I can't CHMOD the permissions to '777' anywhere (ftp, plesk cp etc) I can't find any way of switching the ownership to the usual ftp admin either.

How do I allow myself to upload files to these directories and edit/rename them etc? And how can I stop this happening again?

Thank you for taking the time to read this,