First of all, I would recommend keeping your Showkase and WordPress installations completely separate from each other (in different directories, ie. not storing one within the other).
Having your WordPress installation within your Showkase directory may cause problems if you ever want to move or upgrade either of the programs.
Separating the two from each other would also solve the problem that you are currently experiencing.
In order to have Showkase create a Navigation Link, you would need to ensure that the name of the page does not match any existing page. You could initially call it something other than 'blogg' (just so that Showkase can create the necessary page) and then edit your Showkase pages manually to change the text displayed in the menu. However, please note that such manual modifications will be lost any time your Showkase site is republished.
Alternatively, you could also change the source code of Showkase to add a custom link to the end of your menu (bypassing Showkase's own Navigation Link functionality) by following the example in this forum post.
However, the easiest (and cleanest) solution would be to keep your Showkase and WordPress installations in separate directories.