Hi Benoit,
The difference between the FindAll and Find methods is that the FindAll method returns an array of objects and the Find method returns the first found object in the object tree. You get the error message because you are trying to get the value of the href property of the resulting array. You cannot use the href property in such a way. Here's the corrected script:
Dim PropNames, PropValues, Download_Build
PropNames = Array("innerHTML")
PropValues = Array("Archive Files")
Download_Build = NameMapping.FindAll(PropNames, PropValues, 1000, True)
Log.Message(Download_Build(0).href)
BTW, you can reduce the search time when searching for objects by using the FindAll method of a known parent object.