There have been numerous posts on this issue, but in case you are looking for a solution I have compiled it here.
Problem:
You are working in the SSIS (SQL Server 2005 Integration Services) Business Intelligence Development Studio (BIDS) Control Flow. You are working on a solution that requires that you loop through a list of files. Therefore you add a Foreach Loop to the design surface (below).

You select the "Collection" attribute (left side) and find that the File Enumerator entry is missing (and so is the Item Enumerator).
The problem is associated with an incorrectly set permission on a registry subkey. I found mine associated with a Lexmark key. This issue can be very confusing, particularly when you are operating your SSIS and BIDS as an administrator (as you might on your development computer). The Component Categories Manager is the application that is experiencing this issue.
Solution:
Warning: You will need to reboot your computer after completing these steps. Because you will be modifying the registry, you are advised to back up your registry before proceeding.
1. You will need a program to tell you which registry subkeys are effected. Please follow the steps in the following Microsoft knowledge base article to create this program (written in C#). Compile the program. It will run as a console application.
The title is a little misleading for our particular problem.
Microsoft KB: You receive an error message when you use Business Intelligence Development Studio to build a SQL Server 2005 Integration Services package
2. You must run the program as a user with a "limited account", if running under XP. Do the following steps.
A. Select Start|Programs|Accessories|Command Prompt
B. Change directory to the bin/debug folder of your compiled program.
C. Use the "Runas" command to run the application as a user with a limited account. Here is an example:

When you run the program as this low level user, the following window will appear with a listing of the problem subkeys:

D. Copy the GUID for the subkey.
E. Select Start | Run and in the Open text entry type "regedt32.exe".
F. In the registry editor, locate the HKEY_LOCAL_MACHINE\SOFTWARE\Classes\CLSID folder. From here, search for the GUID(s) you found from running the previous program.
G. When you find the subkey, right-click on it, and select "Permissions...". Add "users" to the list for each subkey that was reported in the previous program.
H. Reboot your computer.
When you open your SSIS project in the BIDS, you should now be able to see and select the File Enumerator.