VOD is ok here. Confirm that you can watch VOD at the Slingbox location using the physical remote control and a TV or monitor connected directly to the DVR. If not, the problem is unrelated to Slingbox.
What goes wrong when using the Slingplayer? (e.g. some button doesn't work, menu doesn't behave correctly, video goes blank, error message, etc.) Provide all details (STB model, viewing device, etc.) If you have another way to view (computer, mobile device, connected device), does that fail the same way?
Thanks for posting in the forums! I'm sorry to hear you are unable to access the on demand feature when connecting to your Slingbox 350.
I am under the assumption that your onscreen remote does not have the button for that. If that is the case, I would recommend reconfiguring your remote to a QIP 6416 (DVR). This remote will have the on demand option and should look just like the remote you use for your actual set-top box.
Below is a link to an article from support.slingbox.com that explains how to change the model of the remote that you're using:
Hope this helps!
The Sling Moderation Team