If you post in an additional reporting currency, click the AssistButton (the one with three dots) in this field to enter the relevant exchange rate in the Change Exchange Rate window that appears.
This is the exchange rate the program will use when you run the Calculate Depreciation or Index Fixed Assets batch job. The resulting FA G/L journal entries will contain this exchange rate. When you post the journal, the program records the entries in both LCY and the additional reporting currency using this exchange rate.