In EM when you right click on a table there are many options available to you.

New Table,

Design Table

Open Table

Full Text

All Tasks

Cut

Copy

Delete

Rename

Properties.

Try all these options out to see what they do. Of course Delete is not a good idea.

But what you want to do is go to the Design Table Option. It will open the table definition in design view, and from there you can add a column to the end of the list, and specify a data type, or you can right click and insert a column between 2 others.

I would also suggest you lear the t-sql equivelent way of doing these tasks.

Look thru books online, or purchase a book like Learn t-sql in 21 days. Not the best book ever, but good for beginners