2 possibilities --
1) if you're talking about in the main search interface
by default we have 'source', 'sourcetype' and 'host' selected in the field picker.
It's possible that maybe you or someone else using the same login, at some point opened the Field picker popup and unchecked source and sourcetype.
To fix, after running your search in the search UI, click the 'Pick fields' link over in the blue sidebar. If source and sourcetype are only on the left side, click them and they'll be added to the selected fields over on the right side.
2) In a dashboard.
If on the other hand you have no idea what Im talking about above, you're probably asking how to do this in a dashboard. It's easy to put events on a dashboard, but i think by default they appear there with no fields, and you have to edit the XML to get them to do so...
So, if that's the case, to fix it click 'edit dashboard', then the 'Edit name/XML' link in the lower left of that panel.
Then a form with some XML will pop up. You'll see a bit that looks like
<event>
<searchString>your search here.</searchString>
<title>your panel name</title>
</event>
add a node to it like so:
<event>
<searchString>your search here.</searchString>
<title>your panel name</title>
<fields>host sourcetype source</fields>
</event>
... View more