I think the the thing that mostly troubles people new to Quicksilver is the plug-in - actions stuff that you have to do to get it to do all the neat things Merlin (and others) do. So, as a quick down and dirty about it here we go:

When you first download Quicksilver it comes with the very basic functionality enabled. There are additional plug-ins that must be installed in order to expand the level of usage beyond application launching. Once a plug-in is installed it opens up (usually) a new range of actions available to the user. Sometimes they are automatically enabled, sometimes you have to enable them through the Actions part of the preferences. Taking Merlin's infamous Text Manipulation plug-in (which isn't a plug-in that is enabled at the installation of Quicksilver) as an example if I wanted to do his append to text trick I would first have to enable (basically download) the Text Manipulation plug-in by checking it's checkbox in the Other (aka Uninstalled) plug-in pane of the Plug-Ins pref pane. You should see something like "Text Manipulation plug-in installed" pop up on your screen letting you know that it has installed the plug-in. The next thing you should do is go to the Preference Pane and click General-->Actions. A (if you have a lot of plug-ins installed) large list of "verbs" or actions comes up and you will have to navigate to the text actions and make sure the one's such as append and prepend are enabled by checking their checkboxes.

This routine needs to happen every time you enable a new plug-in as I notice that some of the cool actions are left unchecked after install and you have to manually turn them on.

Another thing that catches people is the Application Feature Level needs to be set to Beta (this is located in Pref-->Extras).

Hope this helps. :)