the data directory on *nix is
.bitcoin in your home directory. Files starting with a period are hidden. Open a console and
cd ~\.bitcoin"mempool Tx prev not found" errors are from live p2p transactions being received by your bitcoin client, but the blocks that fund those transactions have not been downloaded yet. This is normal from an out-of-sync Bitcoin. A bitcoin transaction has
inputs which are the signatures of the coins that are being sent, and
outputs, which are the addresses where the coins are being sent to. Where it says "FetchInputs failed", the "inputs" that it fails to fetch are the transactions in previous blocks that say that the sender owns the coins.
Hmm I was thinking of setting up a site to HTTPS download the chain for a small cost. Maybe there is merit for it now
And how, exactly, are people supposed to send you coinage when their Bitcoin doesn't work?