There was a change a little while back with the code required to hook into the CamBam menus.
There old code was something like...
MenuItem mi = new MenuItem();
mi.Text = "HeightMap Generator";
mi.Click += new EventHandler(HeightMap_Click);
ui.Menus.mnuPlugins.MenuItems.Add(mi);
This was changed to
ToolStripMenuItem mi = new ToolStripMenuItem();
mi.Text = "HeightMap Generator";
mi.Click += new EventHandler(HeightMap_Click);
ui.Menus.mnuPlugins.DropDownItems.Add(mi);
Plugins containing the old style code will display an error message when you try to load them in later CamBam versions.
There have also been other API changes in subsequent versions which can cause older plugins to break.
Most of these errors will surface when you try to compile the plugins.
Make sure the References section of your .NET project point to the latest CamBam .dlls in the CamBamPlus folder.
In the CamBamPlus\plugins folder, there is also a .zip file containing the source for the heightmap plugin.
This is another good example to look at to help get you started.