DDL Syntax

I am attempting to use the following DDL statement from within Access:

ALTER TABLE MyTableName ADD CONSTRAINT NOT NULL ( MyFieldName1, MyFieldName2 )

and I get a syntax error.  Can someone please tell me what the issue is?

Also, I want to do something like the following

ALTER TABLE MyTableName ADD CONSTRAINT Check ( ( MyfieldName > 0 ) AND ( MyFieldName < 3500 ) )

But this doesn't work either.

I am doing this from the Query functionality within Access.  I have another DDL statement that worked fine:

ALTER TABLE MyTableName ADD CONSTRAINT NoDupes UNIQUE ( MyFieldName1, MyFieldName2, MyFieldName3 )

Any/All help would be appreciated.