The build button is usually disabled when there is an error with the provisioning profile or the app settings. In Visual Studio in the Publish dialog next to the application identifier of your app, you should see a small black icon with an "x". If you hover over that icon, you should see the error detected in your app.
I attached a snapshot demonstrating an app where the application identifier of the app doesn't match the bundle identifier specified in the provision. The App ID associated with the provision is displayed under the Provision drop down in the Publish dialog, so please make sure that your app identifier matches the App ID in the profile if you see such an error on your side.
If the error you get is different, please provide us with more details about what you see on your side - you can attach a snapshot similar to mine so that we can guide you further.