Tabs are something that 1.4.6 doesn't handle very well. They're not what I would call "properly integrated into the UI".
You can apparently manipulate tabs using the rulers but I've never managed to do that successfully myself. (Clicking anywhere in the ruler seems to add a tab stop to the end - no matter where you click - and it's never easy to make adjustments as you sometimes can't see what's happening.) Maybe things are different in 1.5.x or I've just not figured out how to do it properly. Your current best bet - unless you want to experiment - is to use the Tabulators in "PP / Text / Columns & Text Distances".
First, double-click in the paragraph you want the tab to be in (tabs are related to paragraphs, not lines).
Then, go to "PP / Text / Columns & Text Distances" and press the Tabulators button.
Then click on the ruled line (anywhere will do at first) to create a tab stop.
Then change the orientation (top-left drop-down) from Left to Right. (This isn't really necessary here but it's good practice.)
Then drag the tab stop to the desired location (use the document ruler to give you a vague idea of where this should be).
Once done, OK the "Manage Tabulators" dialog.
Put the caret (insertion marker/cursor) at the end of the line where you want to insert the tab and press the TAB key.
Now insert the character you want as your end mark.
If the end mark isn't in the right place, go back to Tabulators and reposition as necessary (this might take some experimentation, and a tab that is too far to the right will move onto the next line). It would be nice if the width of the frame/column was somehow reflected/shown in the Tabulators dialog ruler.
Sometimes, when you go into Tabulators, you might not see the tab stop(s) and you might have to click around in the frame a bit to make things visible.
None of this is easy. Once you get the hang of it it's fairly straight forward - while still being awkward - but it's not exactly user-friendly.