Seeker

1.) Click "Reply to Thread".
2.) Below the box that appears is an option on the right side called "Go Advanced". Select this.
3.) When the page reloads, you will see an icon near the end of the row of buttons atop the text area that looks like a file with a paperclip on it. Click.
4.) A dialog "Manage Attachments" will pop up.* Choose a one or more files using this, and press the "Upload" button.
5.) Close the dialog, and click "Submit Reply".

The instructions are similar if creating a new thread.

* If a different dialog pops up, you might have another uploader selected in your user preferences. To change this, press the "Settings" button at the top right of the screen, and choose General Settings from the list on the left. Towards the bottom of this page is an option called "Enhanced Attachment Uploading", and you can set this to "off". It's mostly up to your personal preference.