Asp.Net pages are "displayed" in a web browser.
Monodevelop is a development environment. If you mean "display in the designer", I have to ask why you're not using Visual Studio (the Express version is free) to develop asp.net web pages. It's got a built-in designer so you can see what your page is going to look like (or at least get an idea of what it's going to look at). If you want to visually design a web page in MonoDevelop, you have to install an add-in. Go here:
http://www.mono-project.com/AspNetEdit[
^]
I would highly recommend, though, that you consider using Visual Studio Express instead.