Google calculator will automatically show up and work when you search valid calculation syntax. eg. 2+2=
If it isn't appearing when you make a mathematical search, you may be entering your syntax invalidly. See https://support.google.com/websearch/answer/3284611?hl=en for more details.
Were you talking about something else which you knew as Google ...
If you're willing to use Bing, you can get it by adding in minutes and seconds to the end of the value.
It even preserves fractional seconds.
(I'm as surprised as anyone that Bing can do something Google doesn't.)
As October 28, 2016, it's still exists
Taken from http://www.google.com/search?hl=en&q=calculator
From the Google Search Help Center
Calculator isn't appearing
If the calculator doesn't show up when you enter in an equation:
Make sure your equation is something that can be computed. For example, if you search for ...
There is no documentation for what functions the Google Search Graphing Calculator can support (see diatribe for the why).
From what I have witnessed, the calculator can only graph three types of functions.
I have only tested the above in 2D space but I believe they also work and 3D spatial coordinates as well.