Subhan,
I am providing a generic answer such that it helps every body, esp. first timers.
Strategy 56, which in one of the automatic forecasting strategies in DP, runs a battery of tests on historical data in the most exhaustive manner. The result of these tests is a model whose forecast results in least MAD. While this may NOT ALWAYS be the BEST FIT model, you are not expected to adopt it without inspection. The eventual model proposed could be any of the constant, trend, seasonal or seasonal trend models. This is by far the MOST exhaustive test for arriving at a forecast model that system thinks is "best fit" but you may not think so, which is fine.
If strategy 56 does not propose seasonal model and YOU THINK it is not right, you can try forecasting with strategy 52 that tests purely for presence of seasonality or 53 that tests for both trend and seasonality or 55 that also tests seasonality.
The first place you should check is the messages tab of interactive forecast screen to SEE the messages you get and the forecast error calculated by the system for each of these auto tests. You will find MAPE in the forecast error tab of interactive forecast screen
If you STILL THINK your history is seasonal, and none of the automatic strategies (tests) reveal positive seasonality, you can directly run forecast using seasonal models 30 or 31 or 35 or 40 or 41 and DO note the forecast error calculated by the system in each case and also the messages. If there is no seasonality, you will see a message like "seasonal test is negative"
The next place you should check is the table of forecast version comparison to compare the results of multiple forecasts ran on your selection.
Now, dump them all to excel and compare what model was "ideal" according to YOU.
Now tabulate all of these errors to know which one indeed offered lowest MAD or MAPE or whatever the measure you selected as forecast error.
You are still not bound by lowest MAD or MAPE as a basis to accept the forecast proposed by the system. You can PICK/ADOPT what you YOU THINK is right.
I hope this gives some directions.
More here
http://help.sap.com/saphelp_apo/helpdata/en/ac/216b6e337b11d398290000e8a49608/content.htm
Thanks
BS