You can find the relevant Gazebo files for the "Office" environment in the virtual machine you downloaded.
The main world file (turtlebot3_mw_office.world) is located in the /opt/ros/melodic/share/turtlebot3_gazebo/worlds folder.
You can copy the whole turtlebot3_gazebo package to a new ROS Melodic installation and launch the world from there:
- Install the standard turtlebot3_gazebo package
sudo apt install ros-melodic-turtlebot3-gazebo
- Copy the /opt/ros/melodic/share/turtlebot3_gazebo from the virtual machine to your other ROS installation in /opt/ros/melodic/share
- Try starting Gazebo with the new world
roslaunch turtlebot3_gazebo turtlebot3_mw_office.launch