msgbox() doesn't give you a lot of options with the formatting. If you want to move the text to the right, you can add an icon.
msgbox(data,title,icon);
Otherwise, you're better off creating your own figure and putting UI elements into it. For example, if you were to create a small figure and put the value and an OK button:
fig = figure('position',[350 350 150 150]);
data = uicontrol('style','text','units','normalized','string',num2str(myData),...
'horizontalalignment','center','position',[.4 .2 .6 .2]);
button = uicontrol('style','pushbutton','units',normalized','string','OK','position',[.4 .1 .2 .1]);
Then, you can just set the button callback to close the figure or something.
