Because you cannot place your cursor in the Body or Title of the Master Slide, it is not possible to directly change the line spacing of the Body or Title in the Master Slide of a Presentation document.
If you wish to change the Master Slide line spacing, you have two options: you can choose the line spacing for the Title or Body on one of the other slides, then copy and paste it into the Master Slide, or you can manually change the Title or Body text on each slide.
To copy and paste from another slide:
1. Go to the slide which has the text formatting that you wish to appear on the Master Slide.
2. Select the Title or Body on that slide and use the Copy command to copy it to the Clipboard.
3. Switch to the Master Slide using the Slide Selector at the bottom of the screen or by choosing Edit Master Slide... from the View Menu.
4. Delete the Title or Body from the Master Slide and use the Paste command to paste in the new Master Slide Title or Body.
To change the line spacing on a single slide:
1. Select all the lines in the Title or Body that you wish to change.
2. Choose Paragraph... from the Text menu. The resulting dialog box will allow you to change the default 1.5 line spacing of the Body or the 1 line spacing of the Title.
Note that this second option overrides the Master slide formatting.