error using horzcat while trying to analyze FC with REST

 Hello, 

I am using REST Functional Connectivity tool to generate the zFC maps and am having this problem:

1) Ultimately, my study will correlate resting state functional connectivity with behavioural scores on a clinical assessment tool....at what point do I add in these behavioural covariates as well as the covariates including age and gender? During the Functional Connectivity step in REST, I originally tried to add in these covariates as a text file (column 1 = age, column 2 = sex, etc...) however I kept getting the error using horzcat...I am not sure what two matrices MAtlab is trying to concatenate....Now I am thinking that the covariate text file option during this Functional Connectivity step is only meant to regress out nuisance covariables such as motion. Is it the case that I only need to attach a list of the rp...txt covariate files?
2) And if this is the case, when can I add in the other covariables?....should I wait until Statistical Analysis for this? 

Thanks in advance, 

Jenna