Sometimes operating system updates change the way you go about things, particularly certain maintenance tasks. OS X is no exception, with the most recent build of the platform, Yosemite, making the task of clearing DNS caches different to previous versions.
Fortunately, Simple Help's Ross McKillop has figured out the new way of doing things. All you need to do is crack open a Terminal window and enter the following command, followed by your user password:
sudo discoveryutil mdnsflushcache;sudo discoveryutil udnsflushcaches
If you were wondering if there's anything else to it, there isn't! As McKillop explains in different post, the mDNSResponder process was responsible for the DNS cache in Mavericks and older iterations of OS X.
How to Clear the DNS Cache(s) in OSX Yosemite (10.10.x) [Simple Help]