I get this error on SS3 with my custom site config

Declaration of CustomSiteConfig::updateCMSFields() should be compatible with that of DataExtension::updateCMSFields()

This is my Custom Site Config

class CustomSiteConfig extends DataExtension {
     
    
    static $db = array(
    	//Business Info
        'BusinessName' => 'Varchar',
        'StreetAddress' => 'Varchar',
        'City' => 'Varchar',
        'Province' => 'Varchar',
        'PostalCode' => 'Varchar',
        'PhoneNumber' => 'Varchar',
        'FaxNumber' => 'Varchar',
        'MainEmail' => 'Varchar',
        'SiteEmail' => 'Varchar',
        //Social Media
        'Twitter' => 'Varchar',
        'FacebookURL' => 'Varchar',
        'GooglePlusURL' => 'Varchar',
        'PinterestURL' => 'Varchar',
        'YouTubeURL' => 'Varchar',
        //About Info
        'About' => 'HTMLText'
    );
    
    static $has_one = array(
    	'Picture' => 'Image'
    );
 
    public function updateFormFields(FieldList &$fields) {
        $fields->addFieldToTab("Root.Main", new TextField("BusinessName"));
        $fields->addFieldToTab("Root.Main", new TextField("StreetAddress"));
        $fields->addFieldToTab("Root.Main", new TextField("City"));
        $fields->addFieldToTab("Root.Main", new TextField("Province"));
        $fields->addFieldToTab("Root.Main", new TextField("PostalCode"));
        $fields->addFieldToTab("Root.Main", new TextField("PhoneNumber"));
        $fields->addFieldToTab("Root.Main", new TextField("FaxNumber"));
        $fields->addFieldToTab("Root.Main", new EmailField("MainEmail", 'Main Contact Email'));
        $fields->addFieldToTab("Root.Main", new EmailField("SiteEmail", 'Email for Contact Forms'));
        $fields->addFieldToTab("Root.Main", new UploadField("Picture"));
        $fields->addFieldToTab("Root.Main", new HtmlEditorField("About", "Sidebar About Copy"));
        $fields->removeByName('Tagline');
        $fields->addFieldToTab("Root.SocialMedia", new TextField("Twitter", 'Twitter User Name'));
        $fields->addFieldToTab("Root.SocialMedia", new TextField("FacebookURL", 'Facebook URL'));
        $fields->addFieldToTab("Root.SocialMedia", new TextField("GooglePlusURL", 'Google+ URL'));
        $fields->addFieldToTab("Root.SocialMedia", new TextField("PinterestURL", 'Pinterest URL'));
        $fields->addFieldToTab("Root.SocialMedia", new TextField("YouTubeURL", 'YouTube URL'));
        
    }
}

Remove the ampersand from updateFormFields(FieldList &$fields).

Okay, that fixed the error, but the Fields aren't showing up. I have the Object declared in my config. Well to clarify, the fields don't show up on my site on my web host (Dreamhost), it works fine only local MAMP stack

I think you should be using "updateCMSFields" not "updateFormFields".

For anyone else who stumbles upon this thread when trying to find out how to add additional site-wide settings (as I did) then I followed the instructions in the link below and it's working perfectly for me (the additional fields are appearing in the "Settings" area in the CMS):

http://doc.silverstripe.org/framework/en/reference/siteconfig

The docs site is offline at the moment, so I cant check..

Has the code that goes in the templates to call the siteconfig values changed? These dont seem to be working for me anymore :(

<h1>$SiteConfig.Welcome1</h1>
<p>$SiteConfig.Welcome2</p>

They have not changed, you still use $SiteConfig.Whatever

Lol.. schoolboy error. I didn't realise I had commented out a bunch of code. I need to add the SS bundle to TextMate for the highlighting syntax