This is How You Can Link AdWords Account to your Google Merchant Login
1) First and foremost, log into your Google Merchant Login (GML)
2) Next, go to Settings, followed by AdWords.
3) The third step is to link the two accounts; to do so copy your AdWords ID and paste it into the Link Account Box
The following image demonstrates these 3 steps:
You can get more step by step information on ‘How to create Product Listing Ads’ here. However, note that it all starts with the linking itself – the very first thing that you will have to accomplish to get the process started is by linking your AdWords and Google Merchant Center account.