Sorry your question hasn't been answered....

I have zero knowledge of this plant or when to harvest seed. Looking at a picture of the seedhead online, I would think that as soon as the prickly "burrs" turn brown and fall off/are easily removed from the stem when you try they are ready.

What is your zone/climate? Cuz' the seeds won't likely be viable if they have been frosted or sitting in the snow and wet for any period of time.

Hopefully someone with knowledge of this plant will come along to help. I thought I would take at least a stab at it since your question has been unanswered for so long ;-) You could also re-post this question on one of the seed forums here on GW, they might have more info.

Ps. not sure if you are intersted, but the link below provides great seed germinating info. Your Acaena is at the top...
CMK

Here is a link that might be useful: Tom Clothier