Since you're going to be boiling the kit (either DME or LME - you still have to boil it) - I'd say just add those hops with 5 minutes left to boil then carry on as usual.
Generally anything added within 15 minutes of flameout will contribute to aroma as the oils responsible won't be wholly lost - as they would be if you were to boil the same hops for 60 minutes.
Here's some good info about hops from John Palmer's How to Brew
about your kit:
I've seen kits like this, and I don't agree with the included instructions. First think about what this kit is - essentially a pre-packaged, half-made beer. Someone (on a large scale) has made the wort already and then dehydrated it to varying degrees. On paper, it should be the beer equivalent of freeze-dried beef wellington - just add some water and you're ready to go.
I don't like this approach for two reasons - lack of sanitation and limited modification potential.
Sanitation
There are a lot of reasons to boil your wort that I won't go into here - if you check out the link I included above you can read all of John Palmer's excellent How to Brew, but I'd really suggest you buy a paper copy. In any case, back to your kit - this is half-made wort that has been dehydrated and then tinned, shelved, shipped out and shelved again until you recently bought it. I have good faith that Munton's is using high quality sanitation practices, but considering how long that can may have sat, a little bit of bacteria could have gone a long way. It's incredibly unlikely that your can of Munton's syrup is contaminated with anything at all - but it pays to be paranoid when it comes to sanitation, and it's an appropriate step to let the beer gods know that you're capable of being their priest...or something. A fifteen-minute boil is enough to kill anything.
Modification
All that being said about sanitation - the other, perhaps more tangible reason, is that boiling opens a world of potential that you're clearly keen to tap into. As the page that I linked to suggests, there's a lot you can do with the same hop depending on how long you leave it in contact with your boiling wort. Beyond hops, however, you can also add sugars or other flavors - check out some of the recipes on hopville. People put all kind of stuff into their beer, some of which is experimental at best, but the point is you can take the same kit of Munton's and come out with vastly different beers if you go down the boiling road.
So here's what I'd suggest:
- Put your can of Munton's in a hot water bath so it's easier to dump out when the time comes.
- Take whatever the largest pot in your house is and fill it with water, less maybe 3L - it should be 2/3 - 3/4 full. Bring it to a boil and add the can of Munton's - be careful not to spill it because it's the devil to clean up. You can dip some of the boiling water out with a ladle and swish that around inside the Munton's can to get all the goo out easier.
- STIR LIKE CRAZY! Don't let anything get stuck to the bottom and burn. It will make your beer taste a bit strange, but more importantly that is an awful thing to try to clean.
- When the water (now wort) has come up to a boil again, the boil has begun. If you are only adding some aroma, I think a 15 minute boil is more than enough. Put your hops into a cheesecloth bag or large tea-ball and throw them in. Be careful of boil-overs at this point.
- When fifteen minutes have gone by, shut off the heat and dump the wort into a food safe plastic bucket (a normal bottling bucket) - if it's food safe, then it's high-temperature safe. You want to expose the wort to oxygen at this point, and doing so will also cool it off considerably. If you have a wort-chiller you don't have to do this. You can fish the hop bag out of the wort at this point using a sanitized spoon or something similar. Careful, it'll be hot!
- Add your water to top it up to 23L (or however much). This will further cool the wort down in addition to diluting it.
- Transfer the wort to whatever you're fermenting in (or leave it in the bottling bucket), wait till the temperature is around 25ºC, pitch the yeast, and put the lid/airlock on.
- Wait 2 weeks and bottle it
