Error message when using get() to return values of OpenMx object

I'm not sure what you mean by "return the values of an OpenMx object". You can save the final state of a model to a file with mxSave or get the free parameters with coef (see ?omxGetParameters). You can evaluate algebras with mxEval and write those values to files with e.g. write.csv. Hopefully this helps. If not, more details please.

you probably don't want to use get(). things inside models are not in the global name space (same reason get(mpg) won't return the mpg column of mtcars dataframe...

You might want mxEval() and mxEvalByName()

So, if we bang out a quick twin model, and interogate it:

require(umx)
data(twinData) # ?twinData from Australian twins.
# Pick the variables
selDVs = c("ht")
mzData <- twinData[twinData$zygosity %in% "MZFF", ]
dzData <- twinData[twinData$zygosity %in% "DZFF", ]
m1 = umxACE(selDVs = selDVs, sep = "", dzData = dzData, mzData = mzData) # -2ll= 9659, a1 = .92
 
#What's A?
mxEval(top.A, model = m1)
            [,1]
[1,] 0.003727328
# or
mxEval(A, model = m1$top)
 
# or
mxEvalByName("top.A", model = m1)

I think Veronica wants mxAlgebraFromString() and then mxEval() that algebra. It it something like this.

astring <- paste0("cbind(", paste0("c", 1:k, "mean", collapse=', '), ")")
# e.g. for k=7 astring is
# "cbind(c1mean, c2mean, c3mean, c4mean, c5mean, c6mean, c7mean)"
palg <- mxAlgebraFromString(astring, name='Jerry')
amodel <- mxModel(... stuff ..., palg)
# ... put palg in your model
mxEval(Jerry, amodel)

Of course, there is another consideration. You may want to re-think your model construction so that you don't need to do this. For example, you could have a matrix like this

amat <- mxMatrix('Full', nrow=k, ncol=1, free=TRUE, labels=paste0('c', 1:k, 'mean'), name='JerryM)

Anywhere you want to use 'c3mean' just refer to it by name. This way you don't need to construct 'Jerry' because you started with 'JerryM'.