How to change solar field aperture area option.

The SAM Help file says:

SAM provides two options for specifying the solar field aperture area: Option 1 (solar multiple) allows you to specify the solar field area as a multiple of the power block's rated capacity (design gross output), and Option 2 (field aperture) allows you to specify the solar field aperture area as an explicit value in square meters.
•Option 1: You specify a solar multiple, and SAM calculates the solar field aperture area required to meet power block rated capacity.
•Option 2: You specify the aperture area independently of the power block's rated capacity.
If your analysis involves a known solar field area, you should use Option 2 to specify the solar field aperture area explicitly.
If your analysis involves optimizing the solar field area for a specific location, or choosing an optimal combination of solar field aperture area and thermal energy storage capacity, then you should choose Option 1, and follow the procedure described below to size the field.

but I can't see how to choose between the two options. I'm using version 2013.9.20 on a MAC (OS 10.9).

I was expecting to see a toggle button or something, but can see anything anywhere.

Thanks,

Matthew

OK, so it looks like the problem is if I try to import a table for the optical efficiency of a generic solar system. In that case, I don't seem to have the choice of Option 1 or Option 2 as mentioned above - that only seems to be a choice if I try to create, say, a parabolic trough physical model. Is that true? Why would that option not be available for the generic solar system with imported efficiency table?

Thank you for clarifying your original question.
The solar field sizing option is not available for the CSP generic solar system model. For that model, you specify the solar field size using the solar multiple. There is not an option to specify the solar field size explicitly in square meters.
There is a mistake in the generic solar system model's Help topic for the Solar Field page -- the "Sizing the Solar Field" section should not mention the two options.
If you want to model a solar field with a specific area, you can use the estimated solar field area value to find the solar multiple equivalent to the area. The equation (from Help) is:
Estimated Solar Field Area = ( Solar Field Design Output + Thermal Loss at Design ) / Total Optical Efficiency * 1,000,000 / Solar Resource at Design
Solar Field Design Output = Power Block Design Gross Output / Reference Conversion Efficiency * Solar Multiple
Best regards,

Paul.