The O's fell behind to a stunning opener from Nick Proschwitz before David Mooney saw a strike chalked off.
Orient then had their second goal of the game disallowed for offside, but Dean Cox eventually got himself on the scoresheet in the 87th minute only for Tom Cairney to steal victory in extra-time.
Slade said: "I've just seen it and it's two yards onside.
"We've done more than enough over the two games to get a result so that just frustrates me even more.
"You want the officials to get it right on the night. You can't have that, it's a massive game for this football club and someone is not doing their job."
- Sports & Recreation
- American Football
- Nick Proschwitz
- David Mooney
- Tom Cairney
- Dean Cox