How do you search the contants of a response message for a string?

Assume my response message has the identifier myMessage.

I have created a variable (myXMLDoc) of type System.Xml.XmlDo***ent
and assigned it as follows.

myXMLDoc = myMessage

I have created a variable (myStr) of type System.String and assigned
it as follows.

myStr = myXMLDoc

I know want to use string manipulation
'XOGResponseState.IndexOf(srchString)' but am getting the message
'distinguished types cannot be referenced with the dot operator'.

Distinguished types are new to me and I haven't had too much success
looking for information on their purpose and use.

Can anyonew explain what is happening here and if I am using the best
method to achieve my goal?

Thanks.