Brad's suggestion to use the built-in number formatting functionality is wise -- it seems like a bad idea to permanently convert a decimal number to a formatted string in a table.
That said, if you really want to do this, read on:
If the Acres field type is Text, you can use the VBScript
FormatNumber function which by default adds thousands separators. Set the first argument to the Acres field and set the second argument to 0 to truncate (or round, not sure which it does) the decimal portion. Be sure to make a backup as this is a destructive operation.
FormatNumber(cint([ACRES]), 0) (note the string to integer conversion)
If the Acres field type is Float or Double, you would have to add another field of type Text and calculate it in a similar way:
Note: It is likely these methods will fail on records with null values so do a select by attributes first to select non-null records, e.g.
"ACRES" IS NOT NULL