Please run the plugin by choosing "Shp to Obs" from the "Vector" menu.
First, a welcome box appears reminding you of the required input files. Continue by clicking "next".
Now a second window opens. It has got four tabs.

In the "Buildings" tab, the buildings layer is selected. After selecting a layer from the upper dropdown menu, the field containing the height of the buildings can be selected from the lower dropdown menu.

The "Trees (optional)"-tab contains the settings for an optional trees layer. If you want to have trees in your obstacle file, select the corresponding trees layer from the upper dropdown menu. Afterwards you should be able to set the fields for the tree's attributes in the lower section.

The third tab is the "Center Point(s)"-tab. The center points layer can be selected from the upper dropdown menu. Afterwards the the field setting the vertical offset of the center points can be chosen by the lower dropdown.

In the "Output Directory"-tab the folder and file name of the new obstacle file(s) is set. If you saved a qgis-project previously, name and path of the project file are proposed.

After selecting the required layers and fields, and entering path and file name, click the "Convert" button to start the converter. According to number and complexity of your input geometries the process may take some time.