You should only need to download the jar; the other files are there just to simplify versioning. 

You will also need to download the apache commons math package and put it in your classpath: http://commons.apache.org/math/

In order to use these add-ons you will have to add them to the class path. The easiest way to do this is to edit your RapidMinerGUI script in your <rapid miner director>/scripts. It's called RapidMinerGUI.bat in windows.

Find the section like this:

## set the path to the files with additional operators here or in an environment variable
#RAPIDMINER_OPERATORS_ADDITIONAL=

Change the last line to be:

RAPIDMINER_OPERATORS_ADDITONAL=/path/to/operators.xml

where the path is where you've saved operators.xml

Next, add the jar to the class path:

CLASSPATH=${RAPIDMINER_CLASSPATH}:${CLASSPATH}:/path/to/RMTTest.jar:/path/to/apache/math.jar

You should now see a new category called "Hypothesis Testing" with the TTest operator in it.