plex.tv/downloads, or the Downloads section at the bottom of this page.
Yes, you want the Intel 64-bit version.
If you are new to Plex on Synology, suggest you read the following two documents. They will save you time and trouble:
Installation & Setup of Plex Media Server on Synology
Synology FAQ (DSM 5 & 6) - Questions, Answers, and a few How-To’s
Edit: If you happen to be running DSM 7, then you’ll need a different package. See this thread:
