object variable is unaccesible in javascript

Try this code:

var cashRegister = {
    total:0,

    add: function(itemCost){
        this.total += itemCost;
    },

    scan: function(item) {
        switch (item) { 
        case "eggs": 
            this.add(0.98); 
            break;

        case "magazine":
            this.add(4.99);
            break;

        }
        return true;
    }
};

cashRegister.scan("eggs");    
cashRegister.scan("magazines");

console.log('Your bill is '+cashRegister.total);

Your mistake was at this line:

total += this.itemCost;

Change the add method to:

add: function(itemCost){
    this.total += itemCost; // "this" was in the wrong place
}

Also, you shouldn't ever use floating-point numbers for money - they're not accurate! Use integers as cents instead. Don't convert to dollars until you need to display the dollar amount. Otherwise you might have magical fractions of cents which can add up over time.

var magicDollars = 1.10 + 2.20;
console.log( magicDollars );  // 3.3000000000000003 - Not good! Money can't materialize itself.

var cents = 110 + 220;
var realDollars = cents / 100;
console.log( realDollars );  // 3.3 - Better. No unexpected fractional cents.