The richtextbox has no built-in search functions. you'll have to treat the contents like a string. what do you want to do with the text once you've found it? if you want to replace it with somethng else, VB has a replace function.
if you just want to find it try something like the following:
find string f in string s:
For i = 1 to len(s)
if mid(s, i, len(f)) = f then 'may want to put ucase's here if you don't eant case-sensitive
do something
end if
next i