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ok so im really new to this game, I've only play the trial version of DOTP2014 when I found this.
In DOTP2014 u pause the combat and cast the instant, but in Forge I cant find the pause button and whenever I click the Instant I what to cast it just blinks and does nothing whatsoever....

would really appreciate any answers

There's no pause button. Each phase will stop unless you've set that phase to skip (You can click on the phase panel to change them from Green to Red, which indicate that phase auto-passes priority.) Please take a screenshot so we can see what your battlefield is like so we can see what the issue would be:

I'm guessing it's one of two issues:
1) You're trying to cast the instant during the declaration portion of a phase, while you are declaring attackers or declaring blockers, the only thing you can do is declare attackers or blockers. Once you've finished declaring attackers/blockers you are allowed to cast instants (this happens before combat damage).
2) Whatever you are trying to cast doesn't have any legal targets.