By default it will comes under in solution explorer of the 
current project

In solution explorer by right clicking the website name 
>>Add Asp.net Folders>>Bin will add the Bin folder

for ASP.net 1.1 its create as soon as u create project or 
solutions

for ASP.net 2.0 framework it use studio 2005. it use its 
own .net web server.... so it is already precompiled code.
so for asp.net 2.0 it create bin folder only when u compile 
or publish.

Right click the project name--->Select Add Asp.Net
Folders--->Select Bin folders in the list of folders

jus right click on ur project ie website name and in that 
list there is a option named add asp.folders..and in that 
option one suboption is provided according to that we can 
add bin folder in asp.net2.0 application

Right click on your WEBSITE name....A list will appear in
which you have to select Add ASP.Net Folder..And when you
keep your cursor on it again one list will appear in which
"Bin" will be there just click on it....And then the Bin
Folder will be added.

I was trying to add the bin folder to my web application
project and none of the proposed solutions above seemed to
work for me...

Until I found out it was already there (that's why I
couldn't add it) but it was hidden because it was not yet in
use. I had to click the "Show all files" button in the
solution explorer to make it appear.