If you have enabled web station in DSM you can install WordPress, Joomla or any similar package. This works great and DSM takes care of setting it up in a correct way. It will also be updated automatically. However you may want to use many instances and by simply adding a wordpress folder you will end up with problems when installing themes and plug ins as the file permissions are incorrect.
First of all you may consider using another server to host your sites. I mainly use my Synology for testing purposes as it is not really built as a webbserver.
Start by downloading the latest wordpress zip file. Put it in the webfolder and unzip it.
Use putty and connect to your nas. Login in as root with the same password you use for admin. You will have to enable SSH in the control panel first. Run the command on your new wordpress site and not the Syn package directory.
Run the following commands:
chown -R http:http /volume1/web/[newwordpress site] chmod -R 775 /volume1/web/[newwordpress site]
I currently run 5.1 beta. It seems like there are some problems with the wordpress package when using this. You may end up with problems related to host url and site url.
In order to setup another WordPress site you will also have to install phpMyAdmin and create a new databases. Make sure to use another name.