You'll want to direct Galaxy usage questions such as this one to galaxy-user@lists.bx.psu.edu (cc'd) so that you can reach a wide audience--including other Galaxy developers--and receive a good response.

I have no experience with megablast, so I won't even venture a guess about what the problem might be.

I would like to use the megablast program, but I don't have the blastdb file.
So, I would like to know how I can create this file and where (what folder).
I have tried to create the file blastdb with two columns separated by TAB, directly on the tool folder, but this failed.

I would greatly appreciate your help.

Best regards