You must install some sort of recovery software to flash any custom ROM / firmware. The one we recommend is ClockworkMod Recovery. Here are the instructions on how to perform ClockworkMod Recovery:
* Power your device off
* Boot into Download-Mode by pressing Volume down + Power
* Hit Volume down to accept the warning
* Start Odin
* Choose “GT-P31XX_ClockworkMod-Recovery_X.X.X.X.tar” as PDA
* Connect your Tablet
* Start flashing
Galaxy Tab 2 7.0 root guide.
To boot into recovery mode, press Volume up + Power button. For navigation:
Volume up: Highlight up
Volume down: Highlight down
Power: Select
Download links for P3100 models:
P3100 (ICS): Download
P3100 (JB): Download
P3110: Download
P3110 (JB): Download
P3113: Download
P3113 (JB): Download