I'd stay away from Milk o Mag. Psyllium husk fiber at Walmart is half the price of Metamucil and is the same thing. If you're not constipated, use psyllium for maintenance only (one tsp) once a day. If you are constipated, take two tsps and one dulcolax. You'll be moving in about six hours. Never take more than one dulcolax while taking psyllium husk.
Watch your diet. Red meat, rice, cheese, dairy, bananas, yams, and anything else that is binding you'll need two tsp or even three tsp just for maintenance. If you get plenty of fiber in your usual diet, you may not need any psyllium.
You want to consume plenty of fluids (not soda pop). You may find that adequate fluids alone will keep you regular.
Important: during and after antibiotics, you need to be replacing the good bacteria in your colon that the antibiotics killed while killing the infection. I use a Multi-Enzyme with Acidophylus in tablet form (get the one that requires refrigeration after opening) and take one tablet with Dannon's "Activia" yogurt (contains bifidus regularus) and Dannon's "DanActive" pro-biotic (contains L casei immunitas). This combination will quickly replace lost bacteria and enzymes that your colon needs to be healthy. Taking these once a day for the rest of your life might ward off any other infections.

So I took Metamucil last night and now I am going all morning. Is that over working my colon or does that mean I really needed the Metamucil? There is some discomfort in my abdomen, but not real pain. I guess this is all working it's way through. This is the first time I've taken the supplement since my attack began 3 weeks ago. Should I take it again tonight to keep things going or should I give my colon a rest?

I would wait a while - I waited 5 weeks AFTER I was done with my antibiotics to start Metamucil. Go easy on the fiber foods too - it takes a while for your body to come back from the trauma of diverticulitis. Stay on softer, easily digestible foods - in the transition from no fiber to slowly adding fiber - I ate a lot of applesauce, bananas, avocado and melon which have some fiber but are easily digested. then started eating steamed and sauteed (well-cooked) spinach and green beans. Also I drink LOTS of water, lots of drinkable yogurt/kefir with live probiotics as well as a Primadophilus capsule (Nature's Way - keep refrigerated!) daily. Your body needs time to heal and calm down before you start very much fiber intake and it takes awhile to get there.
What are you eating at this point and how long have you been off the antibiotics?

I hope I didn't blow it already. I just felt constipated. And the Metamucil did help. What do you do about constipation if you can't have much fiber? I definitely will begin to drink MUCH more water than I have been drinking. Thanks everyone for your help.

For many people psyllium husk takes a little getting used to. As far as constipation goes, my surgeon (after surgery) recommended one to two dulcolax tabs. I've found that one is all I need. Sometimes, if you're not distended with pressure, two shot-glasses of prune juice on an empty stomach will start things moving. It will work in about one hour so don't go anywhere and make sure you don't take too much or you'll be stuck home all day. Also, too much can cause cramping.
Once you're regular, psyllium husk, as I wrote above, is great for maintenance of regularity. It was never much use to me personally as a laxative. It isn't ever too soon to begin fiber but it can be too soon to take too much. Start gradually. It is more important to get and stay regular than to allow yourself to get constipated. Just remember, it's easier to stay regular than to get regular. A good rule of thumb: when bloated with pressure, just stop eating and drink water until the pressure is gone. Then you can begin to worry about how to go about emptying and staying regular.