Sounds like you are dealing with the Stock recovery.
To flash a custom ROM you FIRST need to flash a custom recovery to flash the ROM.
- For CM11 flash Philz Touch or CWM Swipe Edition through ODIN3.
- For OMNI flash TWRP through ODIN3.
This means: Get the .tar.md5 from Android Andi's file repository, install ODIN3 + USB drivers on your computer, put the Tablet into Download Mode, start ODIN3, select the recovery file as a "PDA" file, connect the tablet, flash it, disconnect after flashing, get into the custom recovery (Power + Vol UP), flash the ROM + gapps.
Or ... maybe read the OP post of the custom ROM you want to flash with a bit more care and attention - it's usually pointed out that you need a custom recovery in order to flash the ROM (stock recovery won't cut it).