class User < ActiveLdap::Base
  ldap_mapping :dn_attribute=>"uniqueIdentifier",:prefix=>"dc=stockport,dc=bsfbh,dc=com",
               :classes=>["bsfbhAccount"]
end
ActiveLdap::Base.establish_connection(:host=>"192.168.111.27",:port=>389,:bind_dn=>"cn=root,dc=office",:password=>"secret")

me = User.new('fox')
me.cn = "Testing MOD"
me.save

gives:
/usr//lib/ruby/gems/1.8/gems/ruby-activeldap-0.8.0/lib/active_ldap/base.rb:819:in `method_missing': undefined method `cn=' for #<User:0xb7802aac> (NoMethodError)
        from test2.rb:14

my ldap schema does have the cn attribute.

the example code looks like this:
class User < ActiveLdap::Base
  ldap_mapping :dn_attribute => 'uid', :prefix => 'ou=People',
               :classes => ['person', 'posixAccount']
end
# name, cn and uid are passed via argv
user = User.new(name)
user.add_class('shadowAccount')
user.cn = cn
user.uid_number = uid
user.gid_number = uid
user.home_directory = "/home/#{name}"
user.sn = "somesn"
unless user.save
  puts "failed"
  puts user.errors.full_messages
  exit 1
end