I'm stuck in Wandering in the Dark mission. My food has almost run out and my armor is damaged.
I have repair kits but it will not let me place one in a slot which I presume must be done to use it.

This quests states that I must follow the swallow signs but what exactly are these as I am literally
running around in the dark.

Help would be most welcome.

Two ways to use the repair kits. The easiest way unfortunately is not as reliable as the more cumbersome one, but neither is hard.

Easy...
Drag the repair kit onto an impaired armour or weapon. Note that it need not only be "damaged", but must also show the "damaged icon" on it which is a heavier damage/more impaired state.

Still Easy but not as smooth.
Right click on the repair kit, then choose the target sword/armour piece from the equipped slots.

The amateur kits boost 15% in condition (which means you can't use the "easy" method to maintain the weapon in pristine condition). Journeyman kits boost 40% in condition and Master kits boost 90% of the condition, but you should be *very* careful allowing weapons or armour to degrade so badly, as they lose significant amounts of their effectiveness, making them wear faster and making you more vulnerable... a sword which is worn out too far becomes a 'dead' "Blunt sword" and cannot be restored to it's proper condition... which can be an expensive mistake.

I prefer not to carry too many kits (they are heavy, especially the master kits), 4 amateur, 4 journeyman and 1-2 Master kits seem to be ample.

Instead (or as well as) use the smiths (either armour or weapon smiths can repair either type) and augment the equipment by "using" the grindstones or armour tables to boost their stats by 20% for 15 mintues.. This makes them more effective, and fewer blows from your sword wears it less, and opens you to less attacks as well.

On the walls you will see white chalk animals, (glow red when viewed in witcher senses) there are several different signs shown ~ the swallow, a horse, a dog and a kraken. Later you will also come across carved "witcher medaliion" signs outside caves/buildings indicating the presence of witcher equipment diagrams. These also glow red under witcher senses, and are worth picking (though they may be heavily guarded... check and don't be afraid to run away and come back later if there are "scary monsters" there.

I double click them and then the game highlights the items I can repair with it. Then click the one I need repaired.

I thought weapons didn't actually break? According to Lieste they can...?

It's not necessary for the damage icon to appear on screen to repair the weapon or armor: the icon appears when your weapon or armor has been damaged at least at 50%, but you can repair it even when it is at 60, 73, 87% (for example). As long as it is not at 100%, you can repair it. Of course, repairing your equipment for a damage inferior to the repair kit capacity would be a waste, so try to use them when necessary ;)

Many thanks for the in depth explanation.

I have a further question. Thunderbolt states that its effect duration is 20 seconds so how does one know when this time has elapsed so one can use another? Wikia.com states that its duration is 8 hours.
So what is correct?

20 second base, increased by being "Enhanced" or "Superior", also by some alchemy skills.

The 8 hours would be for the first game on the Aurora Engine.

The remaining times for potions/food/decoctions/poisoning/bleeding/burning etc are shown with named 'counters' in the HUD (Each an icon with text (name and remaining time), and a circumferential tape that counts down the remaining time as a proportion to the maximum.

Thank you. What you say makes sense.

Don't forget that the Superior Tawny Owl potion has the bonus effect of staying active with no timer at night. That is very handy - so much so, that after I get the Superior version, I tend to do my hunting at night.