Dimitar, I actually had this same problem and here is how I resolved it (not an expert by any means).
1. Create your form with the username field inputted into. This form will be on your main page (e.g. the results of the form), but switch it to list mode. In my case it was a list of tasks. I have the list by default in list mode on the one page. The field I use is called "Assigned_to." Make sure there is no space in the name of the column.
2. Create another page in your portal. In my case I called it "My Tasks", which are a list of tasks assigned to the logged in user.
3. A. Go to the top of the admin field and switch the module piece to "Add existing module".
3. B. Select the page that holds your list and module you want to filter by username.
3. C. Select the existing module that you want to filter.
3. D. Add it to the new page. It will look like it does on the previous page in list mode.
4. Go to Form & List Configuration.
5. Scroll down to the FILTER section under Page Related Settings.
6. Put something similar to this (replace Assigned_to with your field name):
[Assigned_to] like '[User:Username]'
7. Click Save Configuration.
This new page will ONLY show data for that form for the logged in user. If you want to use other users though, you'll need to use a querystring with the same copy methodology. I've been only starting to use F&L, but I use it with my customer ID field and create a duplicate of each to tie them all together. In that case, you'll need to use a calculated column with something similar to this:
'<a href="http://dotnetnuke/Customers/TasksbyCustomer.aspx?CustomerCode='+[CustomerCode]+'">Click Here</a>'
In this example, the duplicated task list page I created is setup with a filter to only show rows with the querystring value. Example:
[CustomerCode] like '[Querystring:CustomerCode]'
The value for CUSTOMERCODE that you see in the calculated column is coming from the CUSTOMERCODE in the customer list. Essentially what happens is there's a link on the customer page to the tasks for that customer. The resulting URL that comes in the link looks like this:
http://dotnetnuke/Customers/TasksbyCu...
The only caveat is that when you click on that and there's no data, it will just show a blank page with the title and the search box (if you have it).
Hope this helps. I wish I had this knowledge when I started.