It looks like you changed the width of the rectangle after rounding it. Make sure the rectangle is the size you need before applying rounding.
If you're going to resize a rectangle with rounded corners, make sure to use 9-slice scaling to avoid distortion.
is the 9-slice scaling used to guide and proerly place text evenly in an object?
and lets say that i wanted to have on this toolbar, the current page highlighted... what would be the best way going about that?
No, it's used to eliminate the distortion in the rounded corners of your rectangle. 9-slice can be added temporaily by choosing it from the Scale tools, or it can be added permanently to symbols.
If you want to properly align your text, either use a button symbol witht he text label set to center alignment, or manually center the text using hte align panle or Smart Guides.
Highlighting the button can be done with the button symbol, if I remember corectly, but in best practices for web design these days, it's done using CSS and you'd have to set that up in Dreamweaver or another web editor.