I ran into this situation with the L358. I wanted to set apeture and it would only let me set shutter. This impacts only the transmit methods of the lightmeter.
When you utilize the lightmeter, for either transmit method (RF transmitter, cord), the real data you need to set is shutter speed, not apeture. The very fact that you are triggering your camera(s) via the lightmeter, in essence, means you need to be shooting manual.
You have probably noticed that once you get the reading (at an established shutter speed), you can toggle the lightmeter to find other combinations, based off of that reading.
I have been told to establish a shutter speed of 250 and vary occordingly to achieve the perferred apeture (or calculate it yourself, via stepping down/up stops according). Since shutter and apeture are inverse steps of each other, although annoying, is not too difficult to figure out.
If you are attempting to utilize ETTL, you will find that the lightmeter will trigger off of the pre-flash and it will provide a false reading (it provides you the pre-flash data).