The special Internet@TV account name "develop" is already built into your Samsung firmware. It's intended to enable additional testing functionality for developers of Samsung TV apps. Normally you can create as many individual accounts as you want on your Samsung device (e.g., for family members to each keep their own Facebook password), but the account name "develop" is special. When you create and log into an account with that name, it enables the developer features. It's an internal account, nothing to do with Samsung's internet site. Apparently it's present on all Samsung C and D series TVs and Blu-Ray players with internet app support.
One thing it does is to show an extra menu item under the Internet@TV Settings menu that allows you to specify the IP address of an alternative app store (i.e., not the standard Samsung app store). Developers can set up their own "app store" to download and test their apps during development.
The Plex forum link above gives instructions and the IP address of such an alternative Samsung app server for downloading the pre-built Plex client app for Samsung using this mechanism. The app just appears as a new app added to your existing app list. It doesn't replace anything or make any unusual changes to your Blu-Ray player. It's just an alternative way of getting an app that isn't offered by the Samsung app store yet.
Plex is an effort to develop a high-end media server that runs on PC/Mac/Linux and handles pretty much everything (video/photo/music files, relaying live streams from internet sources like PlayOn does, real-time transcoding etc.). It has an open API allowing the development of plug-ins, and there are hundreds already available. Plex is currently free, but not open-source, and they may have a plan to go commercial eventually.
With the Samsung Blu-Ray player, the Plex server running on your PC is similar to a DLNA server, but on the player end you get a fancy Plex client app that looks like XBMC instead of the simple limited DLNA client offered by Samsung.