I've tried changing the permissions manually on the uploads folder, but it still doesn't activate.
Is that the right folder to change permissions?
Yes. The uploads folder being referred to is the '/wp-content/uploads/' folder within your WordPress installation.
If you have already set the folder permissions to 777 manually, then WP-SimpleViewer should not try to set the folder permissions itself.
It sounds like there may be restrictions on your web server which are preventing WP-SimpleViewer from both checking and settings permissions automatically.
You may have to disable this permission checking and setting routine from the plugin to proceed.
Open the 'wp-simpleviewer.php' file in a plain text editor and delete line 1046:
This will prevent WP-SimpleViewer from quitting after encountering what it thinks is an error which will prevent the plugin from functioning correctly.
Failing that, try deleting lines 1024-1033 (inclusive) which are the lines responsible for the permission checking and setting routine.
Hopefully you will not encounter any more problems and will be able to use WP-SimpleViewer successfully.
SimpleViewer Support Team.