I weld aluminum as a profession.
Follow the guidelines with your Miller machine.
As a base use 190-200 and pedal the amperage to get the base material to puddle FIRST. This should be around 150-175 for non-anodized aluminum plate. THEN add rod to the front of the puddle and work it into the base metal by moving back to the previous "button" on the weld bead and then forward before adding rod once again. Repeat.
Your picture shows a lack of shielding gas coverage: Check you distance from the cup to the base metal AND the CFM of the ARGON gas. It should be 15 or so. IF you're welding outside of a sheltered area with a breeze blowing this WILL greatly affect the shielding gas' ability to cover your weld.
The base metal alloy is a big factor in the choice of filler rod. However as stated by one of the other posters, I would guess that it is a 5xxx alloy.
Either 4043 orm 5356 rod will work.
Stick out, the measure of distance your electrode is past the front edge of the cup, plays an important role as well. while your learning ( and even after) keep the electrode even with the front edge of the cup or no more then an 1/8th of an inch out.
The fillet weld you are attemptinig is generally not given to students of aluminum welding until after they are relatively proficient in the flat and lap welds. Simply put it takes some time to learn and it is pretty tough top figure it all out on your own. But...
Having someone who knows show you can make a HUGE diffence. Check out Miller's website
Miller - Welding Equipment - MIG/TIG/Stick Welders & Plasma Cutting and click on the "Resources" tab. you will find videos that are FREE and they can help you with your technique.
I'll assist you as much as I can via remote control.
Good Luck ! DL