Hi
I also had the same problem and managed to figure it out.
Open up the file nimble-portfolio\skins\default\items.php
Find where it says this (approximately line 43):-
<div class="-link -viewproject <?php echo $readmore_flag ? '' : '-onlyonelink'; ?>">
<a href="<?php echo $item->getData('nimble-portfolio-url'); ?>" class="button-fixed">
Replace it with this:-
<div class="-link -viewproject <?php echo $readmore_flag ? '' : '-onlyonelink'; ?>">
<a href="<?php echo $item->getData('nimble-portfolio-url'); ?>" target=_"blank" class="button-fixed">
Your portfolio URL's will now open in a new window/tab.
Remember that if you update the plugin to a newer version you might have to do this again in case the items.php file gets overwritten with the update. :)