The Shp to Obs plugin needs at least two input files loaded in QGIS (since version 0.1.6 it can be run with a buildings file alone using the layer center as a center point) to create a RayMan obstacle file. All of the required files have to be shape files.
Please make sure all the desired files are in the same metric projection (e.g. UMT, or GK). Degree coordinate systems won't work as RayMan needs the distances in meters.

Buildings file (required): First, a polygon shape file is required, that contains the buildings for the .obs. It needs a field in the attribute table stating the height of each building. This field must not contain any missing values.

Center points file (optional for version >= 0.1.6): Second, a point shape file is required, setting the center point(s) of the .obs file to create. If this layer consists of several points, Shp to Obs will create an obstacle file for each of the points. The center points input layer requires a field in the attribute table, that specifies the vertical offset of each point. A negative value in this field will increase the height of all obstacles, while a positive value will reduce the height respectively. Zero means no offset. This field must not contain missing values.

Trees file (optional): Optionally, Shp to Obs is able to include trees into the obstacle file. Therefore a third input layer is necessary. The trees layer has to be a point shape layer and must possess at least the following five fields, that, again, must not have missing values.

? A height field specifying the total height of the tree (in m).
? A crown radius field, that gives the radius of the crown expansion (in m).
? A trunk height field that sets the distance between the ground and the lower side of the crown (in m).
? A trunk radius field setting the radius of the trunk (in m).
? A type field specifying the type of the tree (1 char in small letters, "l" for a leaf tree, or "n" for a coniferous tree).