I tried with substituting your "distributor" with account1: 

    var request = require('request');
    var StellarSdk = require('stellar-sdk');
    var server = new StellarSdk.Server('https://horizon-testnet.stellar.org');
    StellarSdk.Network.useTestNetwork();
    
    //account1
    pub1='GCBCTFOR2GES3JI5SKCYSDP7ZRIB523WQXZ5QC5FWXSK2FEMFMLJULIT'
    sec1='SB44SZ5IQCK72ELARVZ53BLSPO4NFCKH7H6ZBVGIFZ47CWLD5O2ADCOD'
    kp1=StellarSdk.Keypair.fromSecret(sec1)
    
    //account2
    pub2='GDNICVTW44YSI7BD6XB6GKPGCO6H367XT3JUFXG3XAFW46SOVKCBMPAI'
    sec2='SDIC6OCFXYLEBCJQIEDN5LN2KNVW4VZYY6FEGTIQ5CCSU7KLCPHRGIH4'
    kp2=StellarSdk.Keypair.fromSecret(sec2)

    var distributor=new Object    //just create Obj
    distributor.publicKey=pub1
    distributorKeyPair=kp1
    
    var MCoin = new StellarSdk.Asset('MCoin', pub1);
    var XLM = StellarSdk.Asset.native();
    
    server.loadAccount(distributor.publicKey).then(function(account){
            var transaction = new StellarSdk.TransactionBuilder(account)
                    .addOperation(StellarSdk.Operation.manageOffer({
                        selling: MCoin ,
                        buying: XLM,
                        amount : '200',
                        price: 2,
                        offerId: 0,
                    }))
                    .build();
            transaction.sign(distributorKeyPair);
            //console.log(transaction.toEnvelope().toXDR('base64'));
    
            server.submitTransaction(transaction)
                .then(function(transactionResult){
                    console.log(transactionResult);
                    console.log('\n Success!')
                })
                .catch(function(err){
                    console.log('An error has occured');
                    console.log(err);
                })
    
        }).catch(function(e){
            console.log(e);
        })


No problem spotted. Maybe could you verify if your `distributor.publicKey` gives a string of publickey & `distributorKeyPair` is an account object? 




    > distributorKeyPair
    Keypair {
      type: 'ed25519',
      _secretSeed: <Buffer 79 c9 67 a8 80 95 fd 11 60 8d 73 dd 85 72 7b b8 d2 89 47 f9 fd 90 d4 c8 2e 79 f1 59 63 eb b4 01>,
      _secretKey: <Buffer 79 c9 67 a8 80 95 fd 11 60 8d 73 dd 85 72 7b b8 d2 89 47 f9 fd 90 d4 c8 2e 79 f1 59 63 eb b4 01 82 29 95 d1 d1 89 2d a5 1d 92 85 89 0d ff cc 50 1e eb ... >,
      _publicKey: <Buffer 82 29 95 d1 d1 89 2d a5 1d 92 85 89 0d ff cc 50 1e eb 76 85 f3 d8 0b a5 b5 e4 ad 14 8c 2b 16 9a>,
      publicKey: 'GCBCTFOR2GES3JI5SKCYSDP7ZRIB523WQXZ5QC5FWXSK2FEMFMLJULIT' }
    
    >
    >
    > distributor.publicKey
    'GCBCTFOR2GES3JI5SKCYSDP7ZRIB523WQXZ5QC5FWXSK2FEMFMLJULIT'



p.s. I am using NodeJS v7.    What version are you using btw?