Quick solution
Move the focus to the file browser (⌥⌘⇥) and show invisibles (⌥⌘i)(key names: option/alt+command/windows+i/eye). All invisible files will appear in the file browser. Press ⌥⌘i again to hide them.
Permanent solution
If you want to always show some dotfiles (for instance .htaccess), you can either add them in Preferences > Projects > Include files matching…
or add the list of files you want to show to the include array in one of:
~/Library/Application Support/TextMate/Global.tmProperties(same as using Preferences)~/.tm_properties(global).tm_properties(inside a specific directory/project)
You can add all dotfiles (.*) but I don’t recommend it since it can be a lot of clutter.
That said, you can also exclude files with the exclude array.