It's not clear to me if you're talking about a slide margin or margins within
text placeholders. Do you want all the text to be centered? If so, you can do
that in the slide master (View>Master>Slide Master). Click the text and click
Center on the Formatting toolbar. If individual slides were changed, you may
need to reapply the layout. Choose Format > Slide Layout. Right-click the
layout you want and choose Reapply Layout.
You wouldn't normally set margins with a right tab but if you choose View >
Ruler, you'll see a ruler and it has a gray, right margin that you can drag.
Again, to affect the entire presentation, do it in the slide master.
You can set margins between the text and the edge of a placeholder.
Right-click the placeholder (in the slide master, for example), and choose
Format Placeholder. Click the Text Box tab and set the internal margins.
However, most text placeholders don't have visible borders, so you won't see
a margin, but this setting will affect how the text wraps.
Ellen