Trying to use an IF statement to add months to a DATE in a formula field

Trying to use an IF statement to add months to a DATE in a formula field

Hi All, 

Hoping you can help, I am using a formula to return a date field, but I want it to be conditional.  Keep in mind that the Orders.Day Supply field is a dropdown. 

I've tried it this way:

if(${Orders.Day Supply}=30)
{
Adddate(${Orders.Subscription Start Date},1,'MONTH')
}
else if (${Orders.Day Supply}=60)
{
Adddate(${Orders.Subscription Start Date},2,'MONTH')
}
else if (${Orders.Day Supply}=90)
{
Adddate(${Orders.Subscription Start Date},3,'MONTH')
}
else if (${Orders.Day Supply}=120)
{
Adddate(${Orders.Subscription Start Date},4,'MONTH')
}
else if (${Orders.Day Supply}=150)
{
Adddate(${Orders.Subscription Start Date},5,'MONTH')
}
else if (${Orders.Day Supply}=180)
{
Adddate(${Orders.Subscription Start Date},6,'MONTH')
}
else
{
now()
}

And 
If(Tonumber(${Orders.Day Supply}, 30))
{
Adddate(${Orders.Subscription Start Date},1,'MONTH')
}

With both always returning:
Your string is either missing its quote marks, or it needs a comma separating it from another value.
If you can provide guideance that would be greatly appreciated.
Crystal