I suspect you get an error because moving down one table is not valid in the current selection context (is there more than one table?).
Unfortunately, I find manipulating word selection programatically infuriating as Microsoft documentation is severely lacking in details. You end up finding something that works by trial and error.
Is it really from table to table that you want to move, rather than table cell to table cell? If the latter, I'd get the table cell without involving selection and set the selection to the content of that cell:
word = actxserver('Word.Application');
selection = word.Selection; %acquire selection reference
%...
table = wdoc.Tables.Item(n); %where wdoc is your word document, and n the index of the table
table.Cell(row, column).Range.Select; %move selection to specific row and column of table
selection.TypeText('Hello world!') %do something with selection
If it's table to table, I'd still use the same principle (index the table collection, get selection from table object).
