'Effective' is directly related to a players stamina. As players play their effectiveness drops, from a starting point of 100%. When setting a players effectiveness, you're telling the SIM at what level you want a tiring player taken out of the game to rest on the bench.
'Distribution' works differently for QBs/RBs and WRs/TEs.
For QBs and RBs it determines what % of the time a player will be the ball carrier on run plays. For example, in I-form if you set your QB at 0, RB1 at 20 and RB2 at 10, your QB will carry the ball 0% (0/30) of the time, RB1 will carry the ball 66.6% of the time (20/30) and RB2 will carry the ball 33.3% (10/30) of the time. For WRs/TEs it is similar, but determines only who your QBs 'first read' will be. Ie who's the first player your QB looks to after hiking the ball. It's important to note that it only effects the QBs first read. Setting a WR/TEs distribution to 0 will not prevent your QB from passing to them.
For more information I suggest reading past developer chats:WIS Developer Chats
For this particular issue, check out one of the more recent ones:GD v2.0 - Part 1