I've been reloading shotshells on the MEC 600 Jr. for over 30 years and currently have 4 of them set up in the 4 skeet gauges (12, 20, 28, .410). They may have changed the design a bit over the years since I bought my presses, so bear in mind that what follows applies to the older models, though I doubt that things have changed much.
The standard 12 gauge target load uses 1-1/8 ounces of [lead] shot. MEC charge bars are set up to drop a fixed charge of shot from the shot charge hole (labeled "S") and either (1) have a fixed size hole for powder (I believe this is only on older charge bars), or (2) use a bushing in the powder charge hole (labeled "P", 'natch) to vary the powder charge according to what powder you're using. Do not under any circumstances ever use a charge bar designed to use bushings without the correct powder bushing in place.
The charge bar should have numbers stamped on the end of the bar at the shot side. Bars that take bushings have "302" on top [the MEC part number] and the weight of the shot charge on the bottom. In the case of my 12 ga. press, the bar has 302 and 118, indicating that the bar is drilled to drop 1-1/8 ounces of shot. On the 28 ga. it's 302 and 034 (or maybe just 34, I forget), indicating 3/4 oz. of shot, the standard 28 ga. target load. Both of those bars need a bushing installed on the powder side. My 20 ga. has a fixed powder charge bar, which is labeled "20 ga 2A" and I have the MEC chart that indicates a 2A charge bar is set up for 7/8 oz. of shot, the standard 20 ga. target load. There's also a chart that shows the weight of perhaps two dozen different powders that each bushing is drilled for.
Instructions for safely choosing a load, setting up, and using the press are a bit more than I can get into here, but if you give me the numbers on the ends of your charge bars I can tell you what they're set up for and we can go from there. Note that shotshell loading is different from metallic handloading in that you don't, or shouldn't, "experiment" - you need a book of safe loads that is based on what hulls you'll be working with and you should follow them exactly. I use "The Handbook of Shotshell Reloading" by Cougar and Henderson, but there are others.
I also have the factory user manuals for the press, which I'd be glad to copy and send to you, along with the bushing and charge bar charts, or you can download all that and more from the MEC web site here:
http://www.mecreloaders.com/
