Ok first of all every one has to pay AHV and tax for income in Switzerland.
As a freelance there are two opportunities.
1. You are self employed (Einzelunternehmung)
2. The law do not accept your self employed and the company who has given you the work has to look for the AHV to be paid and send you an income statement or take the tax amount direct from your salary (depends on permit you have)
In your case I think that for AHV the company who has given you the work has to pay for you.
More detail about "working black in Switzerland" can be found at
http://www.keine-schwarzarbeit.ch
The form you received is to give the details about your work to see if you count as self employed or as a normal worker. So fill it out and send it in and everything is ok.
Consider that for self employment (Einzelunternehmung) you will also have to pay MwSt. if you have a turnover higher then 75K. And below 100K a year turnover no entry into the HR (Handelsregister) has to be made