@suryarobertson it is not clear to me if you want to choose tickers in AmiBroker and then plot a MACD based on parameters data that you "dynamically" get from Excel (with AmiBroker controlling Excel automation) or you want to simply select a cell in an Excel sheet and then update a chart in AmiBroker.
Using Excel Automation can be a little tricky. To get you started if you have access to the old Yahoo group see this discussion thread and study its attachments.
For the second scenario, you could arrange your MACD parameters in contiguous cells (where each row will have the ticker in column A, the Fast period in col B, the Slow in C and the Signal in D).
Then, each time a cell is selected in column A (at row x, in the range where you have the tickers), you could trigger some VBA code to write the appropriate (Bx, Cx, Dx) row data to a file (like a short "include" .afl file) and then update the main ticker in the AmiBroker active chart.
How to do it was explained in this past thread. The OLE update will also trigger a re-evaluation of the include file.