Index: trunk/src/org/openstreetmap/josm/gui/preferences/SourceEntry.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/preferences/SourceEntry.java	(revision 6151)
+++ trunk/src/org/openstreetmap/josm/gui/preferences/SourceEntry.java	(revision 6152)
@@ -159,4 +159,5 @@
     /**
      * Returns the parent directory of the resource inside the zip file.
+     *
      * @return the parent directory of the resource inside the zip file,
      * "." if zipEntryPath is a top level file; null, if zipEntryPath is null
@@ -167,5 +168,9 @@
         File dir = file.getParentFile();
         if (dir == null) return ".";
-        return dir.getPath();
+        String path = dir.getPath();
+        if (!"/".equals(File.separator)) {
+            path = path.replace(File.separator, "/");
+        }
+        return path;
     }
 }