I suspect that the problem is caused by invalid steamid sent in the SELECT query (there is some checking in register_client to see if the steamid is correct, but there is none in Load_MySQL).
1. SQL_NumResults returns number of rows retrieved from database. If you execute SELECT ... WHERE steamid = '%s'; it will be greater than 0 for all steamids that are present in database so "if" condition will be false.
2. There is nothing wrong with SELECT * FROM ...
3. Using SQL_ReadResult to check if there were any rows retrieved from database is incorrect.