That is the wrong location to download Plex Media Server for Synology.
Step 1: Read Installation & Setup of Plex Media Server on Synology
and Synology FAQ (DSM 5 & 6) - Questions, Answers, and a few How-To’s
Step 2: Download the current PMS version from Plex: https://www.plex.tv/media-server-downloads/#plex-media-server. The DS1515+ has a Intel 64-bit CPU, so download the Intel 64-bit package. See FAQ #3.
Step 3: Perform a manual installation as outlined in the support documents.
