Can't help you with the MAC part, but basically, to post an ad here, go down to the Trading Post section, there are three sub sections there.
One is for guns only, one is for gun parts, accessories and ammo reloading components and equipment, etc, , and one is for all non gun related items. They appear in that order.
Pick the appropriate sub section, and left click on it to open it up.
At the top you should see a blank box, if you left click on it, a blank ad will appear. Fill in the title, starting with WTS, which stands for Want To Sell, or WTB, if it's something that you are looking to Buy, or WTT, for Want to Trade for something.
Fill in the location with the name of the city you live in.
Then fill in the details of what it is you want to sell, description, condition, price.
Then drag and drop the pictures of the actual item you are offering for sale, ( no stock or file photos, has to be a picture you took of your sale item) into the ad.
Then just click the box marked post thread at the bottom of your ad and you're done.