The folder arrangment ought to look like this if you have multiple campaigns and are using the SCP's "MOD" option
If you don't have a tables folder you can do two things. You can either just dump the .tbl folder into the "data" folder, it should be read, but the better thing to do is to just create a "tables" folder in the "data" folder, and dump it in there because that's the proper dissemination and that's the way it works.
If you're not using the MOD option from the SCP launcher, then you're probably using the main "data" folder (located in the freespace2 folder, looking like freespace2/data/tables) so any .tbl files should go in there.
Hope that clears it up. I'd give you screenshots but alas, not installed at the moment.