What version of FileMaker are you using and what method are you using to filter the portal?
You can filter portals at the data level by including your drop down field as part of the relationship or (FileMaker 11 only) you can define a portal filtering expression that filters the portal. I need to know which method you are using to get the details right when I answer.
I'm using Filemaker Pro 11 Advanced and am using its portal filtering option.
Then, if you add an "all" value to your search field's value list, you can modify your portal filter expression to be something like:
YourTable::SearchField = "All" or //put your current expression here.
You can also add a "find all" button to your layout that sets the search field (or even a separate global variable) to "All" and then refreshes the window to update the filtered portal. That sometimes is easier than getting your value list to include "All" if its values are drawn from a table in your database--though it is possible to do that if you want.
Just realized I forgot to acknowledge PhilModJunk's help in this problem. Thanks for the help, all is well. :)