Hi,
No need to edit your meta.tpl. Are you using the CDSEO Pro version? If so, you can easily add any category meta description through the Administration panel>Categories and then pick the category you wish to edit. Then you can submit the category meta description and keywords etc.
If you are using CDSEO basic you can easily go into admin, go to categories, pick the category you wish to edit and then choose the button "modify category". Then edit the field META description below.
Don't forget to clear your browser cache and/or templates cache after submitting the changes. If you do forget you might not see any changes you made..
Good luck,