[cybio]

1.) floatbox.js and the other files of floatbox have to be put in the folder "..wp-content/plugins/floatbox-plus/floatbox/" and not "...wp-content/plugins/floatbox-plus/" (just put the whole floatbox folder of the downloaded zipfile of floatbox 3.51 in the floatbox-plus folder).
on the options page of floatbox-plus, there should be a warning, if it doesn't find the files in the expected place.

2.) there is no need to add something to the header.php manually, delete them...

if there is no more warning on the options page of floatbox-plus, then everthing should be fine...