Hi David,
To add EEG positions from a built-in template, you must not select a channel file and instead populate the UseDefault parameter. The correct process call would look like the following:
% Process: Add EEG positions sFiles = bst_process('CallProcess', 'process_channel_addloc', sFiles, [], ... 'channelfile', [], ... 'usedefault', 49, ... % ICBM152: BioSemi 128 A1 'fixunits', 1, ... 'vox2ras', 1);
Best,
Martin