what error msg is this?

The error message is just what it says: it can't find the method you requested. What version of BIG-IP are you using? Per the documentation on that API method:

http://devcentral.f5.com/wiki/default.aspx/iControl/System__SystemInfo__get_all_cpu_usage_extended_information.html

Click here

That method was introduced in BIG-IP v9.4.0. If you are using an older version, you cannot make use of that method since it's not on that version.

Question 2:

As for exception handling, the calls you are making are straight into the .Net runtime. To catch the exception you need to wrap the calls with a try/catch

try { 
   m_interfaces.SystemSystemInfo.get_all_cpu_usage_extended_information();  
 }  
 catch (Exception ex) 
 { 
   String sEx = ex.Message.ToString(); 
   // Log or display error here. 
 }

This same behavior would occur if you tried to use any of the other .Net calls and they return an exception. You have to catch it or it will crash the app.

-Joe