Identify chromosome location from gene nucleotide or amino acid number
Option 1: You can use the SNP database, to see if it has (and it usually is) characterised before: The tutorial is here (http://www.sciencebuddies.org/science-fair-projects/project_ideas/BioMed_Bioinformatics_NCBIGene_SNP.shtml)

Option 2: is to use a go to character function in a few text editors. If you are on windows, You can do it in Notepad++ (https://superuser.com/questions/487507/how-can-i-find-nth-character-in-notepad); if in Linux/mac, you probably know how to do it in vim (https://stackoverflow.com/questions/22862558/go-to-a-specific-offset-in-a-document). If using other text editors in Linux, ask around that community, and they'll tell you how to go about it.

Option 3: One another labour intensive but easy way to do it is to use the tools available online. I personally use NCBI for this, and play around with the region shown. This prevents you from counting. Once you get the local region, select a  few nucleotides  to the left and right, and use that as a search term in your sequence (find text and replace text)



Option 4: And my personal favourite, get the computer to do it for you like this: https://stackoverflow.com/questions/23662748/replace-a-nucleotide-at-a-certain-position-in-a-dna-sequence-file

If you are using IGV, it is said in the documentation here (https://www.broadinstitute.org/igv/Navigate) on how to search for regions and even insert mutations. Were you not able to make this work ?