Index: /trunk/src/VBox/Frontends/VirtualBox/src/globals/VBoxProblemReporter.cpp
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/globals/VBoxProblemReporter.cpp	(revision 30995)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/globals/VBoxProblemReporter.cpp	(revision 30996)
@@ -1597,4 +1597,19 @@
 }
 
+void VBoxProblemReporter::remindAboutGuestAdditionsAreNotActive(QWidget *pParent)
+{
+    message (pParent, Warning,
+             tr("<p>The VirtualBox Guest Additions do not appear to be "
+                "available on this virtual machine, and shared folders "
+                "cannot be used without them. To use shared folders inside "
+                "the virtual machine, please install the Guest Additions "
+                "if they are not installed, or re-install them if they are "
+                "not working correctly, by selecting <b>Install Guest Additions</b> "
+                "from the <b>Machine</b> menu. "
+                "If they are installed but the machine is not yet fully started "
+                "then shared folders will be available once it is.</p>"),
+             "remindAboutGuestAdditionsAreNotInstalled");
+}
+
 int VBoxProblemReporter::cannotFindGuestAdditions (const QString &aSrc1,
                                                    const QString &aSrc2)
Index: /trunk/src/VBox/Frontends/VirtualBox/src/globals/VBoxProblemReporter.h
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/globals/VBoxProblemReporter.h	(revision 30995)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/globals/VBoxProblemReporter.h	(revision 30996)
@@ -307,4 +307,5 @@
                                    const QString &, const QString &);
 
+    void remindAboutGuestAdditionsAreNotActive(QWidget *pParent);
     int cannotFindGuestAdditions (const QString &aSrc1, const QString &aSrc2);
     void cannotDownloadGuestAdditions (const QString &aURL, const QString &aReason);
Index: /trunk/src/VBox/Frontends/VirtualBox/src/runtime/UIMachineLogic.cpp
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/runtime/UIMachineLogic.cpp	(revision 30995)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/runtime/UIMachineLogic.cpp	(revision 30996)
@@ -1457,4 +1457,6 @@
     /* Show shared folders settings dialog: */
     UISharedFoldersDialog dlg(defaultMachineWindow()->machineWindow(), session());
+    if (!uisession()->isGuestAdditionsActive())
+        vboxProblem().remindAboutGuestAdditionsAreNotActive(defaultMachineWindow()->machineWindow());
     dlg.exec();
 }