Your problem's coming from this block
php
$insert = "INSERT INTO user (Username, Password) VALUES '".$_POST['username']."', '".$_POST['pass']."')";
$add_member = mysql_query($insert);
You're telling it to put the data into columns 'Username' and 'Password'. However, from the previous query
php
$check = mysql_query("SELECT username FROM user WHERE username = '$usercheck'")
You are using the column name 'username'. If your column names are all lowercase, as they should be, then you need to change your column names in your insert query to use lowercase letters.
Example:
php
$insert = "INSERT INTO user (username, password) VALUES '".$_POST['username']."', '".$_POST['pass']."')";
That should fix your problem, assuming the column names and everything is correct.
NotesA few notes:
1. Do not put $_POST variables directly into your queries. I know you used addslashes() but you should store your $_POST variables in another variable, and run something like mysql_real_escape_string().
2. Make sure you name your variables things that make sense. Variables like $check and $check2 do not help us at all. For instance, $check2 should be named $numRows or something to that effect, since it holds the number of rows returned from the database. This will make your code clearer for others reading it, like ourselves.
Just some friendly suggestions. Hope this helps.