So you've created a Tabular Note and saved it as a Custom Symbol? I don't think the Custom Symbol will separate the text as a separate and editable entity since it is more or less embedded in the Tabular Note itself. I'm fairly certain you'd have to draw the table with lines and place the text manually to get the desired outcome you're seeking (a Custom Symbol with editable text). That's how we used to have to do such tasks prior to the addition of Tabular Notes.
If it were me, and I'm not claiming this to be the best or most elegant solution, I'd just create the Tabular Note and save it in a standalone file. When needed, do a File -> Import Part and import the saved file with the Tabular Note into whatever drawing file requires this table. Once the table is imported you can drag it to where you want it or use the Move command to be more precise with the placement.
Tim Flater
NX Designer
NX 9.0.3.4 Win7 Pro x64 SP1
Intel Xeon 2.53 GHz 6GB RAM
NVIDIA Quadro 4000 2GB