[rospack] Error: stack/package stage_ros not found; in ROS Fuerte, Ubuntu 12.04

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Hi, all. I am very new to ROS and Ubuntu. I am working under Ubuntu 12.04 with ROS Fuerte. I was trying to make teleop_base tutorial with stage (Simulating one robot). After the teleop_base installation I tried to run:

$ rosrun stage_ros stageros $(rospack find stage_ros)/world/willow-erratic.world

And I have got this error:

[rospack] Error: stack/package stage_ros not found [rospack] Error: stack/package stage_ros not found

I was trying to find out how solve it in the internet (I assume, that I need to install stage_ros), but I did not find anything useful. At least anything that I can understand and use (as I am newbie). Please help me!