spectopo.m from EEGLAB:
%%%%%%%%%%%%%%%%%%%
% for your epoched data, channel 2
[spectra,freqs] = spectopo(EEG.data(2,:,:), 0, EEG.srate);

% delta=1-4, theta=4-8, alpha=8-13, beta=13-30, gamma=30-80
deltaIdx = find(freqs>1 & freqs<4);
thetaIdx = find(freqs>4 & freqs<8);
alphaIdx = find(freqs>8 & freqs<13);
betaIdx  = find(freqs>13 & freqs<30);
gammaIdx = find(freqs>30 & freqs<80);

% compute absolute power
deltaPower = mean(10.^(spectra(deltaIdx)/10));
thetaPower = mean(10.^(spectra(thetaIdx)/10));
alphaPower = mean(10.^(spectra(alphaIdx)/10));
betaPower  = mean(10.^(spectra(betaIdx)/10));
gammaPower = mean(10.^(spectra(gammaIdx)/10));
%%%%%%%%%%%%%%%%%%%



absolutepower.m
%%%%%%%%%%%%%%%%%%%
% Mean power
% Takes data from channels 1:14, computes mean power across the dataset,
% outputs data by frequency band
%%%%%%%%%%%%%%%%%

for j = 1:14
% for data, channels 1:14, all frames
[spectra,freqs] = spectopo(EEG.data(j,0.001:EEG.pnts,:), 0, EEG.srate);

% delta=1-4, theta=4-8, alpha=8-13, beta=13-30, gamma=30-60
eegchannel(j).deltaIdx = find(freqs>1 & freqs<4);
eegchannel(j).thetaIdx = find(freqs>4 & freqs<8);
eegchannel(j).alphaIdx = find(freqs>8 & freqs<13);
eegchannel(j).betaIdx  = find(freqs>13 & freqs<30);
eegchannel(j).gammaIdx = find(freqs>30 & freqs<40);

% compute absolute power
eegchannel(j).deltaPower = 10^(mean(spectra(eegchannel(j).deltaIdx))/10);
eegchannel(j).thetaPower = 10^(mean(spectra(eegchannel(j).thetaIdx))/10);
eegchannel(j).alphaPower = 10^(mean(spectra(eegchannel(j).alphaIdx))/10);
eegchannel(j).betaPower  = 10^(mean(spectra(eegchannel(j).betaIdx))/10);
eegchannel(j).gammaPower = 10^(mean(spectra(eegchannel(j).gammaIdx))/10);


end
% concatenate mean power by channel
deltaPowerAllChannels = [eegchannel(1,1).deltaPower eegchannel(1,2).deltaPower eegchannel(1,3).deltaPower eegchannel(1,4).deltaPower eegchannel(1,5).deltaPower eegchannel(1,6).deltaPower eegchannel(1,7).deltaPower eegchannel(1,8).deltaPower eegchannel(1,9).deltaPower eegchannel(1,10).deltaPower eegchannel(1,11).deltaPower eegchannel(1,12).deltaPower eegchannel(1,13).deltaPower eegchannel(1,14).deltaPower];
thetaPowerAllChannels = [eegchannel(1,1).thetaPower eegchannel(1,2).thetaPower eegchannel(1,3).thetaPower eegchannel(1,4).thetaPower eegchannel(1,5).thetaPower eegchannel(1,6).thetaPower eegchannel(1,7).thetaPower eegchannel(1,8).thetaPower eegchannel(1,9).thetaPower eegchannel(1,10).thetaPower eegchannel(1,11).thetaPower eegchannel(1,12).thetaPower eegchannel(1,13).thetaPower eegchannel(1,14).thetaPower];
alphaPowerAllChannels = [eegchannel(1,1).alphaPower eegchannel(1,2).alphaPower eegchannel(1,3).alphaPower eegchannel(1,4).alphaPower eegchannel(1,5).alphaPower eegchannel(1,6).alphaPower eegchannel(1,7).alphaPower eegchannel(1,8).alphaPower eegchannel(1,9).alphaPower eegchannel(1,10).alphaPower eegchannel(1,11).alphaPower eegchannel(1,12).alphaPower eegchannel(1,13).alphaPower eegchannel(1,14).alphaPower];
betaPowerAllChannels = [eegchannel(1,1).betaPower eegchannel(1,2).betaPower eegchannel(1,3).betaPower eegchannel(1,4).betaPower eegchannel(1,5).betaPower eegchannel(1,6).betaPower eegchannel(1,7).betaPower eegchannel(1,8).betaPower eegchannel(1,9).betaPower eegchannel(1,10).betaPower eegchannel(1,11).betaPower eegchannel(1,12).betaPower eegchannel(1,13).betaPower eegchannel(1,14).betaPower];
gammaPowerAllChannels = [eegchannel(1,1).gammaPower eegchannel(1,2).gammaPower eegchannel(1,3).gammaPower eegchannel(1,4).gammaPower eegchannel(1,5).gammaPower eegchannel(1,6).gammaPower eegchannel(1,7).gammaPower eegchannel(1,8).gammaPower eegchannel(1,9).gammaPower eegchannel(1,10).gammaPower eegchannel(1,11).gammaPower eegchannel(1,12).gammaPower eegchannel(1,13).gammaPower eegchannel(1,14).gammaPower];

%concatenate again to create a single matrix where X is channel and Y is
%frequency ascending
absolutepowermatrix = [deltaPowerAllChannels; thetaPowerAllChannels; alphaPowerAllChannels; betaPowerAllChannels; gammaPowerAllChannels];

%%%%%%%%%%%%%%%%%



Modification to pop_chanedit.m file from EEGLAB:

(Line #:163)
Original: if isempty(chans) || ~ishandle(chans)
Edited: if isempty(chans) || ~max(ishandle(chans))