You can use TextFrame to add text and set its formatting, fonts etc.
Below is the C# code to do so, please also see the output presentation attached. It also gives you idea, how to use Aspose.Slides to change formatting of text.
C# Code
Presentation pres = new Presentation();
Slide sld = pres.GetSlideByPosition(1);
//Adding Title with textframe
Aspose.Slides.Rectangle rect = sld.Shapes.AddRectangle(432, 384, 4896, 720);
rect.LineFormat.ShowLines = false;
TextFrame tf = rect.AddTextFrame("Your Title Text");
//Center align it
tf.Paragraphs[0].Alignment = TextAlignment.Center;
//Set formatting
Portion port = tf.Paragraphs[0].Portions[0];
//To set font height
port.FontHeight = 44;
//To set font
FontEntity newFont = new FontEntity(pres, pres.Fonts[0]);
newFont.FontName = "Comic Sans MS";
int newFontIdx = pres.Fonts.Add(newFont);
port.FontIndex = newFontIdx;
//To set color
port.FontColor = Color.BlueViolet;
pres.Write("c:\\outTitle.ppt");