Can I override the depreciation method or years to depreciate assets? I can't get them to match what my accountant entered last year.

[Spino]

In the asset/depreciation dialogue, the first question is did you buy any items for $2500 or less. This will enter items as expenses rather than assets. There follow two other questions. One is about claiming a special depreciation allowance for assets entered late in the year. The other is about improvements (that you might be able to expense.) Finally you get to a table labeled "Your Property Assets".
SCROLL DOWN (in case it doesn't show on your screen, depends how many assets you already have entered.)
AT THE BOTTOM OF THE TABLE is a button labeled "Add an Asset". That's it! This will lead you to questions that place the asset in a class, etc.....

SELECT "OTHER ASSET" type to customize years, class, method, etc. manually.
[If you're using the desktop software you also have the option to go to forms, got to the schedule E wks for the property, click on depreciation, click the magnifying glass to the left, and "create a new copy" to add an asset worksheet for an asset not yet listed. This is especially useful if you are moving to TTax and have a list of assets already in service to enter, as you describe.]

NOTE: You say you are using TurboTax Free Edition. I do not think you will have easy success with what you are trying to do in that edition. You will find the "Business" edition is most tailored for you, and I do recommend you use the download version to your computer rather than the online version. Generally people with rentals have more complex returns, which is also suggested by the fact that your trying to enter assets already in service in the way that an accountant previously has. One tip to consider--if you have a pdf of your last year's tax, TT may be able to import a lot of information from that. It's worth a try. Use the one you have with the most forms and worksheets, not just what you file if you have a choice.

Also, bringing something over from an accountant can be a bit more complicated, it might be worth finding a consultant (either paid or friendish) if you're not yet comfortable with everything on your return. Once you get used to it and have done it one year, future years are MUCH easier.

[Capital]

What is the difference in depreciation method SL (my accountant uses) and Turbotax's default SL/MM?

The yearly depreciation is different between these two methods. Its for commercial real estate rental.

[DaveF1006]

The difference is that in the month when the asset was first place into service.  For SL/MM, the asset started depreciating on the 15th of the month when it was placed into service. For the SL, it started depreciating on the actual day on the month it started depreciating. 

The difference in overall depreciation shouldn't be that much different though unless there are some other underlying considerations. 

@Capital