Installation of turtlebot_simulator fails with apt-get

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Hello all, I am facing issues to install the turtlebot_simulator stack with the command:

sudo apt-get install ros-indigo-turtlebot-simulator

I get the following error:

Install these packages without verification? [y/N] Err http://packages.ros.org/ros/ubuntu/ trusty/main ros-indigo-amcl amd64 1.12.7-0trusty-20160420-064058-0700
      404  Not Found [IP: 64.50.233.100 80]
Err http://packages.ros.org/ros/ubuntu/ trusty/main ros-indigo-voxel-grid amd64 1.12.7-0trusty-20160419-215226-0700
      404  Not Found [IP: 64.50.233.100 80]
Err http://packages.ros.org/ros/ubuntu/ trusty/main ros-indigo-turtlebot-stdr amd64 2.2.2-0trusty-20160420-100701-0700
... 
      404  Not Found [IP: 64.50.233.100 80]
Err http://packages.ros.org/ros/ubuntu/ trusty/main ros-indigo-turtlebot-simulator amd64 2.2.2-0trusty-20160420-101710-0700

First question: How can I solve this problem?

Second question: How can I install the turtlebot_simulator stack from source? I read tthe related documentation but I was not able to do it from source. Is it possible to someone give a step by step instructions for this? Suppose I got the source codes with:

git clone https://github.com/turtlebot/turtlebot_simulator.git

and now?