When I run the following command:
ALTER staff ADD status char(1)

mySQL returns the following error:

Error on rename of '.\Common\staff.MYI' to '.\Common\#sql2-6a-2fc3.MYI' (Errcode: 13)

What is the problem?

Thankyou.

Problem is between chair and monitor :-)))
Now seriously. Problem is in the syntax of command.
The right command is following:
alter table staff add column status char(1);

More at url: http://www.mysql.com/doc/en/ALTER_TABLE.html

I tried it exactly as above and till gives me the same error.

Hmmm... this seems to be a bug in the MySQL product.

OK
1.What version you are running?
2. What Operating system you are runnig?

OS = Windows 2000
mySQL = 3.23.47-nt

Solution:
http://www.mysql.com/doc/en/ALTER_TABLE_problems.html

I read the man page but can not find any a- or b- files. I did notice that my error code was different from the example on the man page.

Mine is error: 13 and the example is error: 17. Were can I find the meaning of these codes?