You need to provide the full path and file name to
Assembly.LoadFrom(..)[
^], e.g.:
Assembly assembly = Assembly.LoadFrom(@"c:\test\MyAssembly.dll");
But you only provided a path (or, if "plugin" is the name of the file, you have to append ".dll" or, unlikely but possible, ".exe").
Edit after comment:
To load all assemblies from a specific directory, you can use
Directory.EnumerateFiles(..)[
^] to retrieve all (matching) files from the directory and then loop over the
IEnumerable<string>
-result to load them one by one. (The linked page has a sample code.)