The answer must be obvious to the ArcGIS community, but it is not to newcomers like me. In case anybody ends up being stuck like I was, here is the solution I found after reading a couple of ArcGIS tutorials.
In order to use the freehand tool in Arcgis 10.1, you need to create a shapefile (.shp) first. This can be done in ArcCatalog : browse to the folder containing your data, right click on the folder (from the Catalog tree) then select New > Shapefile from the contextual menu.
The "Create New Shapefile" dialog box opens. The freehand tool will generate polygons, so choose Feature type : Polygon. Define the spatial reference with Edit, when all set press OK.
Go to ArcMap, add the newly created shapefile to the layer. Then you can start Editing (Editor toolbar, accessible from Customize > Toolbars > Editor). Click Editor > Start Editing then again Editor > Editing windows (second to last in the contextual menu) > Create features. A Windows will pop-up with a list of editable layers. Select the layer corresponding to the shapefile you want to edit, then the Construction tools appear down in the Create Features windows. Pick the Freehand tool and voilà !
Suggestion to ESRI
The learning curve for ArcGIS is pretty steep, I guess it puts off a number of potential users to actually consider ArcGIS as a solution. There could be a wizard to make things easier and faster for the user in cases like this. For example, let us assume the user wants to edit a raster image with the freehand tool. A single button could do the following operations automatically :
- create a SHP file associated to the raster image
- get the same spatial reference as the raster image and the correct Feature type
- start editing windows and Feature creation
