In PHP, the character for newline is "\n" (must be double quotes).
After a few hours of fiddling, I think the problem is in the MODX ExtJS code.
The newlines are there in the DB as you said. They're also there when the data is pulled from the DB. They're even there after the call to $modx->getChunk(). So they're there in the page output when MODX renders it.
I'm guessing somewhat, but the JS that renders the page in the Manager has the resource fields hard-coded into it. The textarea fields are specified with an xtype of 'textarea'. Because the ExtJS code that renders the page doesn't know that our field is a textarea (even though its HTML is exactly the same as that for the introtext and description fields), it uses the default xtype of text. I believe it strips out any newlines when rendering it. Short of using a custom class and some fancy JS code that somehow convinces ExtJS that these are fields with an xtype of textarea (and I'm not sure that's possible), I don't see any solution.
I may be able to modify the plugin code to insert br tags with nl2br(), but it's not a great solution, and the lines would still appear on one line:
line1<br />line2<br />line3
I think the br tags would persist.
Would that help?