If you are using the cycle charging (CC) or load following (LF) controllers, you will need to create a "large, free" converter. This is because these controllers do not support batteries on the AC bus. To create a large free converter, add a converter to your HOMER model, set the converter search space to 0 and a large number greater than the amount of power you would expect to transfer between the buses (such as 9999999). Then, set all of the costs in the cost matrix to $0. You can now add a battery and HOMER will treat it like it is on the AC bus, except that it will account for efficiency losses for transferring energy from the AC to the DC bus (i.e rectification) and from the DC to the AC bus (i.e. inversion). Under this approach, you will want to make sure that your battery includes the costs for any equipment to interconnect the battery to the AC bus.
If you are using the Generator Order Dispatch controller or developing your own controller using one of the HOMER APIs, you can create an AC battery directly in the HOMER UI. There is no need to add a "large, free" converter (unless you would like to compare results with the LF or CC controllers). To include an AC battery with these controllers, add a controller that support AC batteries, and when you add a battery a new option will appear to put the battery on the AC bus in the battery component. You will want to ensure that your battery includes the cost for any AC/DC conversion internal to the component, as well as account for any efficiency losses in the battery's roundtrip efficiency.