During the season i plan to use my 31 year old RB a lot.
Should i give him a lot of minutes in the pre season or sit him the entire preseason? And why is his conditioning low? http://mfn20.myfootballnow.com/player/1447

He is older so more prone to injuries so I would either sit him or play him 3rd on depth chart.

I am not sure on the conditioning but he should be all set by the time season starts

All players have a low conditioning at the start of the season, it's higher if they attend training camp but they do need actual game time to get fully into game shape. So it's up to you to try to balance giving him enough playing time without risking injury.

I have found that pulling starters after:

game 1: 1st quarter
game 2: 2nd quarter
game 3: 3rd quarter
game 4: 1st quarter

seems to have the best balance of getting your players conditioned without risking too much injury to your starters.

So If I have my players unavailable(non active) then they will skip training camp? If so, what are the benefits of skipping a training camp?

No benefit for missing camp.
They must be active and in the Depth Chart at their positions, I believe.

I don't think the depth chart has any bearing on Training Camp.

This is correct. The only thing to make sure for TC is that your players are at the positions (their assignment that shows up on your roster view, not the depth chart) that you want to use them primarily. You can go through TC with all players inactive and no one in your depth chart and all players will receive the same benefit as if they were all active and in the depth chart. You don't have to have the depth chart set until your first game (although of course having it set before TC may help you decide to move a player, say, from RDE to LDE before TC, which you _do_ want to do if that's where he'll be playing this year)