Installation manual: mono-project.com
Forum: UbuntuForums.org

The System.Windows.Forms.dll is contained in the mono-winforms2.0-cil package. The file resides in /usr/lib/mono/2.0/. To make it show up in MonoDevelop, you might have to create a file /usr/lib/pkgconfig/windows-forms.pc with the following contents:

prefix=/usr
exec_prefix=${prefix}
pkglibdir=${exec_prefix}/lib/mono/2.0

Name: System.Windows.Forms
Description: Windows Forms for Mono
Version: 2.0.0.0

Libs: -r:${pkglibdir}/System.Windows.Forms.dll

Afterwards you can add this as a Reference to your project in MonoDevelop.