This is very tricky, because the authors are using this expression in a special sense (and it occurs elsewhere in the work of Maturana in particular). "Trae a la mano" seems to be used sometimes in Chilean Spanish like "trae de la mano" (conlleva), as here, for example:

"todo lo que el niño o niña trae a la mano es su experiencia previa"
http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0718...

Another use is approximately "contribute" or "put forward"; this example actually comes from a blog post about Varela and Maturana:

"En ese marco teórico, quiero traer a la mano algunas ideas que nos permitan profundizar en aquella intuición tan bella sobre la posibilidad de cultivar las ideas y la Historia."
http://www.hernandinamarca.cl/?p=782

But here it means something a little different. This passage can be found in English, in an essay by a professor of industrial design called Gui Bonsiepe:

"In this respect Maturana and Varela write:
The nervous system does not «capture information» from the environment, as one often hears, but the opposite: it brings a world within reach by specifying which configurations in the environment are disturbances and which changes are triggered by these in the organism."
http://tdd.elisava.net/coleccion/5/bonsiepe-en

That is an option, and it certainly makes sense. But I don't think it's exactly what they mean.

Varela himself explains that the expression "traer a la mano" really means "enactuar". Here is a very enlightening passage:

"La palabra enacción fue acuñada, además, por el científico chileno Francisco Varela, “La enacción: una alternativa ante la representación. Tanto en el cognitivismo como en el conexionismo, el criterio del conocimiento sigue siendo una representación adecuada de un mundo externo que está dado de antemano. [...] Este enfoque es incompleto, porque la mayor capacidad del conocimiento en los seres vivos consiste en plantear las cuestiones relevantes que van surgiendo en cada momento de nuestra vida. Estas cuestiones no están predefinidas sino que son enactuadas: se las hace emerger desde un trasfondo [...] corrientemente enacción se usa en el sentido de traer a la mano o hacer emerger, que es lo que me interesa destacar"
http://bibliotecadigital.academia.cl/bitstream/handle/123456... (pp. 62-63)

And so it has been interpreted by commentators on Maturana and Varela:

"Varela y Maturana dirían que el ser humano enactúa (trae a la mano) un mundo (Maturana y Varela, 1990, 1998; Varela y otros, 1997)."
"Una mente generosa para un mundo sistémico", R J Herrera
(from the Google results page; the article won't open).

And in English:

"enactivist and embodiment literature proposes the alternative of a subject that enacts a world (Maturana & Varela, 1980)"
https://books.google.es/books?id=THSsAgAAQBAJ&pg=PA52&lpg=PA...

"This leads Maturana and Varela to conclude that the nervous system does not simply represent a world; rather, it creates, calls forth, or enacts a world."
https://books.google.es/books?id=NiCCAgAAQBAJ&pg=PA26&lpg=PA...

I think you could use either of these English terms, or indeed both.

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Note added at 2 hrs (2015-04-24 11:40:37 GMT)
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The point is that "brings a world within reach" implies that it gives us access to a world that already exists independently outside us, but what Varela and Maturana are referring to is closer to the idea of calling a world into being.

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Note added at 1 day11 hrs (2015-04-25 21:17:02 GMT) Post-grading
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Thank you, Susana. This was a fascinating question and I very much engaging with it. Best regards!

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Note added at 1 day12 hrs (2015-04-25 21:45:12 GMT) Post-grading
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I can't even type a thank-you note without making a mistake :) I meant to say that I very much enjoyed engaging with it.