If you receive this error then ensure you've inserted the full namespace into the ResourceManager constructor as in the example below:
System.Resources.ResourceManager rm
= new System.Resources.ResourceManager("My.Name.Space.StringResources", typ...
If this doesn't rectify the problem, take a look through your Project folder in Windows Explorer and find the .resources file. I found that mine had "Properties" appended before the filename, because it was in a subfolder called Properties (the VS.IDE put it there). So my code went something like this:
SysRes.ResourceManager rm = new SysRes.ResourceManager("My.Name.Space.Properties.StringResources", typ...
Hope this helps! Luke