I think this may be the meaning of "sea interesada" in this specific context. My reading of the text with a possible (loose) translation (that is certainly susceptible to improvements and corrections) would be:

c) Identificación… del convenio arbitral o acuerdo específico que vincule a las partes y del que a su vez se deduzca o derive la sumisión a la Corte –aportando en su caso copia de los mismos-, salvo que dicha sumisión ***sea interesada *** en la propia solicitud de arbitraje a resultas de su aceptación por el demandado o demandados

c) Identification… of the specific arbitration agreement or clause binding the parties and from which submission to this court may be inferred –providing a copy thereof if applicable--, unless submission (to this court) ***is implicit*** in the request for arbitration itself, in the event that it has (already) been accepted by the respondent or respondents

Would this work?

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Note added at 4 hrs (2014-10-12 13:46:23 GMT)
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Yes, I think the idea is that if the respondent or respondents have already responded to a request for arbitration filed at that court, then this is tantamount to admitting submission to arbitration without having to provide evidence of a previous arbitration agreement. Providing evidence of an arbitration agreement isn't necessary if by their conduct they have already shown their willingness to submit to arbitration.