To enable most .plgx-Plugins, you need to install the mono-gmcs-Package. For newer versions of KeePass, use mono-dmcs. For a list of all needed packages, look at leo.gaggl.com.
Installation manual: mono-project.comForum: 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: Afterwards you can add this as a Reference to your project in MonoDevelop.