Ok, first let´s make sure You have put the plug-in in right place and got it working.
Actually divide scanned images is a script (.scm) so it belongs to the script folder.
Download it here: http://registry.gimp.org/node/22177
Your scripts folder is located at your Home folder/.gimp2.8/scripts. Before putting it there, right-click it, select properties, click "unblock this yadayada...", apply, ok.
It´s under Filters->Batch Tools.
Scanning in Gimp:
File menu Create->Scanner/Camera. This will give You a pop-up to select source, select your scanner from the list (the uppermost option). It will launch your scanner software, preview the flatbed and scan. It will be transferred automatically to Gimp.