I have added kuka kr-150 before and i will try to explain
You need to click "Create Mechanism" under "Modeling" tab and then change your mechanism type as a "Robot"
Double click "link" item and select your base and click make enable "set as baselink". Double click "link" item again and select other axis...
After insert your all axises, double click to "joint" item then select your parent link as a base and child link as a first axis.
Make this process for all axises but you should be careful because all motors are standing different location and we have to define. (you can check robot specification files to find these info). Also joint limits are defining here. I just wrote random number for limitations.
After create all joints, double click to "frame" item. This area is not so important for simple simulations. This is for robot coordinate system.
Your robot is ready for joint movement like mechanism. If you want you can simulate your robot but your robot can not make linear movement now.
There is a folder "mediapool" under robotstudio files. We should find .cfg files of one robot. Then copy, paste and change information with your new robot specification.
I have attached some pictures to help on this post.