Use SRP to refactor this Ruby Class
To start out with all of the 

require './lib/numeral_adders/&lt;lib&gt;


should be moved to a separate file like/lib/number_interpreter 

Moving on, looking at repetitive parts of your code. The add_numerals method has a lot of lines like this:

@numeral &lt;&lt; &lt;type&gt;_adder.add_numerals.split('')


and you have methods like this:

def &lt;type&gt;_adder
  &lt;type&gt;Adder.new(@num_interpreter.&lt;unit&gt;)
end


You can make this DRY by enumerating over a list defined elsewhere. For example:

# in the lib/number_interpreter.rb file
class NumberInterpreter
  Adders = {
    MAdder =&gt; :thousands,
    CMAdder =&gt; :hundreds
    # ...  
  }
end

# in the main file (numeral_adder.rb)

def add_numerals
  NumberInterpreter::Adders.each do |klass, unit|
    @numeral &lt;&lt; adder(klass, unit).add_numerals.split('')
  end
  flatten_numeral
end

def adder(klass, unit)
  klass.new(@num_interpreter.send(unit))
end


Important concepts here include setting a class as a hash key then initializing it using new later on. It also uses send for programmatic method invocation. You're writing a generic adder class to make all that repetition unnecessary - its arguments are a class to initialize and a unit to convert the results with. 

By the way your flatten_numeral is so simple that it'd be better to not make it a separate method. The flatten would probably be unnecessary if you use concat instead of &lt;&lt; (shovel):

@numeral.concat adder(klass, unit).add_numerals.split('')