1. Create a new java project in eclipse. Call it MyCalculatorProject or something along those lines.
2. Right click on src file inside of your project and then click import.
3. Select "File System"
4. Next to "From directory:" click "Browse" and find the folder that has the calculator file you downloaded from github.
5. Select the checkbox next to the java file.
6. Click finish.
7. Make sure Calculator.java is inside of the src folder and default package.