You're half way there.
Now that you've created the variable that stores the path for Play, you need to add that variable to your system's path for it to be accessible from the command line.
In the same list of system variables (where your new variable resides), find the 'Path' variable, and add the variable name you created for the Play framework location, surrounded by %.
For example, if your new variable is called Plat (as your screenshot shows), then you'd edit your Path variable to look something like this:
[previous path element];%Plat%
Keep in mind that the semicolon separates variables in your path, so if you need to add additional variables, be sure to use the semicolon on both ends of your variable.
Just as an FYI, you can also skip the step you've already done (creating a new environment variable) and simply add the path to your installation of Play directly to the Path:
[previous path element];C:\webapp\Play
Both implementations will achieve the same result; creating a variable first just simplifies the re-use of the path in other variables.