Attaching a MDF with a missing LDF
When a database is detached, it should shut down cleanly. This file has been shadow copied or has been taken from a crashed SQL Server Instance or such. That is, the MDF is not from a cleanly detached, shutdown database.

So, possible solutions..


I don't think this will work thought but please check. Try SP_ATTACH_SINGLE_FILE_DB from a query window. It is clearer than the SSMS GUI attach pop-up. Note, this is deprecated in favour of CREATE DATABASE .. FOR ATTACH_REBUILD_LOG which states as a requirement: A clean shutdown of the database.
But I'm sure that you'll have to go back and state you have a corrupted file.
Better still, ask for a BACKUP which will be smaller and consistent. 
If your source says they can attach it, they'll be lying or losing data.
Looking at SO and SF, there appears to one approach that will cause data loss. You can find that for yourself: I don't endorse it.