How to get + metacharacter for sed in Terminal?
OS X's implementation of sed uses the "basic" regular expression format by default, which doesn't support +.  From man re_format:

Obsolete (``basic'') regular expressions differ in several respects.  `|'
is an ordinary character and there is no equivalent for its functional-
ity.  `+' and `?' are ordinary characters, and their functionality can be
expressed using bounds (`{1,}' or `{0,1}' respectively).  Also note that
`x+' in modern REs is equivalent to `xx*'.  The delimiters for bounds are
`\{' and `\}', with `{' and `}' by themselves ordinary characters.  The
parentheses for nested subexpressions are `\(' and `\)', with `(' and `)'
by themselves ordinary characters. [...]


So, what can you do?  Well, one possibility is to tell sed to use the modern format with the -E switch.  This may, however, force you to rewrite other parts of your RE to conform (for example, swapping \( ... \) with ( ... )).  Another option is to use one of the suggested alternate forms above.  Here are examples using the various approaches to match "A" followed by one or more "B" or "C", followed by "D":

sed -E 's/A[BC]+D/[match]/'
sed 's/A[BC]\{1,\}D/[match]/'
sed 's/A[BC][BC]*D/[match]/'